Add url to "Interpret the call graph" section in pprof legend. (#530)

internal/driver/testdata/pprof.contention.cum.files.dot

 digraph "unnamed" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "Build ID: buildid-contention" [shape=box fontsize=16 label="Build ID: buildid-contention\lComment #1\lComment #2\lType: delay\lShowing nodes accounting for 149.50ms, 100% of 149.50ms total\l"] }
+subgraph cluster_L { "Build ID: buildid-contention" [shape=box fontsize=16 label="Build ID: buildid-contention\lComment #1\lComment #2\lType: delay\lShowing nodes accounting for 149.50ms, 100% of 149.50ms total\l\lSee https://git.io/JfYMW for how to read the graph\l"] }
 N1 [label="file3000.src\n32.77ms (21.92%)\nof 149.50ms (100%)" id="node1" fontsize=20 shape=box tooltip="testdata/file3000.src (149.50ms)" color="#b20000" fillcolor="#edd5d5"]
 N2 [label="file1000.src\n51.20ms (34.25%)" id="node2" fontsize=23 shape=box tooltip="testdata/file1000.src (51.20ms)" color="#b23100" fillcolor="#eddbd5"]
 N3 [label="file2000.src\n65.54ms (43.84%)\nof 75.78ms (50.68%)" id="node3" fontsize=24 shape=box tooltip="testdata/file2000.src (75.78ms)" color="#b22000" fillcolor="#edd9d5"]

internal/driver/testdata/pprof.contention.flat.addresses.dot.focus.ignore

 digraph "unnamed" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "Build ID: buildid-contention" [shape=box fontsize=16 label="Build ID: buildid-contention\lComment #1\lComment #2\lType: delay\lActive filters:\l   focus=[X1]000\l   ignore=[X3]002\lShowing nodes accounting for 40.96ms, 27.40% of 149.50ms total\l"] }
+subgraph cluster_L { "Build ID: buildid-contention" [shape=box fontsize=16 label="Build ID: buildid-contention\lComment #1\lComment #2\lType: delay\lActive filters:\l   focus=[X1]000\l   ignore=[X3]002\lShowing nodes accounting for 40.96ms, 27.40% of 149.50ms total\l\lSee https://git.io/JfYMW for how to read the graph\l"] }
 N1 [label="0000000000001000\nline1000\nfile1000.src:1\n40.96ms (27.40%)" id="node1" fontsize=24 shape=box tooltip="0000000000001000 line1000 testdata/file1000.src:1 (40.96ms)" color="#b23900" fillcolor="#edddd5"]
 N2 [label="0000000000003001\nline3000\nfile3000.src:5\n0 of 40.96ms (27.40%)" id="node2" fontsize=8 shape=box tooltip="0000000000003001 line3000 testdata/file3000.src:5 (40.96ms)" color="#b23900" fillcolor="#edddd5"]
 N3 [label="0000000000003001\nline3001\nfile3000.src:3\n0 of 40.96ms (27.40%)" id="node3" fontsize=8 shape=box tooltip="0000000000003001 line3001 testdata/file3000.src:3 (40.96ms)" color="#b23900" fillcolor="#edddd5"]

internal/driver/testdata/pprof.cpu.flat.functions.call_tree.dot

 digraph "testbinary" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "File: testbinary" [shape=box fontsize=16 label="File: testbinary\lType: cpu\lDuration: 10s, Total samples = 1.12s (11.20%)\lShowing nodes accounting for 1.11s, 99.11% of 1.12s total\lDropped 3 nodes (cum <= 0.06s)\l" tooltip="testbinary"] }
+subgraph cluster_L { "File: testbinary" [shape=box fontsize=16 label="File: testbinary\lType: cpu\lDuration: 10s, Total samples = 1.12s (11.20%)\lShowing nodes accounting for 1.11s, 99.11% of 1.12s total\lDropped 3 nodes (cum <= 0.06s)\l\lSee https://git.io/JfYMW for how to read the graph\l" tooltip="testbinary"] }
 N1 [label="line1000\n1s (89.29%)" id="node1" fontsize=24 shape=box tooltip="line1000 (1s)" color="#b20500" fillcolor="#edd6d5"]
 N1_0 [label = "key1:tag1\nkey2:tag1" id="N1_0" fontsize=8 shape=box3d tooltip="1s"]
 N1 -> N1_0 [label=" 1s" weight=100 tooltip="1s" labeltooltip="1s"]

internal/driver/testdata/pprof.cpu.flat.functions.dot

 digraph "testbinary" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "File: testbinary" [shape=box fontsize=16 label="File: testbinary\lType: cpu\lDuration: 10s, Total samples = 1.12s (11.20%)\lShowing nodes accounting for 1.12s, 100% of 1.12s total\l" tooltip="testbinary"] }
+subgraph cluster_L { "File: testbinary" [shape=box fontsize=16 label="File: testbinary\lType: cpu\lDuration: 10s, Total samples = 1.12s (11.20%)\lShowing nodes accounting for 1.12s, 100% of 1.12s total\l\lSee https://git.io/JfYMW for how to read the graph\l" tooltip="testbinary"] }
 N1 [label="line1000\n1.10s (98.21%)" id="node1" fontsize=24 shape=box tooltip="line1000 (1.10s)" color="#b20000" fillcolor="#edd5d5"]
 N1_0 [label = "key1:tag1\nkey2:tag1" id="N1_0" fontsize=8 shape=box3d tooltip="1s"]
 N1 -> N1_0 [label=" 1s" weight=100 tooltip="1s" labeltooltip="1s"]

internal/driver/testdata/pprof.heap.flat.inuse_space.dot.focus

 digraph "unnamed" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: inuse_space\lActive filters:\l   tagfocus=1mb:2gb\lShowing nodes accounting for 62.50MB, 63.37% of 98.63MB total\l"] }
+subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: inuse_space\lActive filters:\l   tagfocus=1mb:2gb\lShowing nodes accounting for 62.50MB, 63.37% of 98.63MB total\l\lSee https://git.io/JfYMW for how to read the graph\l"] }
 N1 [label="line2001\n62.50MB (63.37%)" id="node1" fontsize=24 shape=box tooltip="line2001 (62.50MB)" color="#b21600" fillcolor="#edd8d5"]
 NN1_0 [label = "1.56MB" id="NN1_0" fontsize=8 shape=box3d tooltip="62.50MB"]
 N1 -> NN1_0 [label=" 62.50MB" weight=100 tooltip="62.50MB" labeltooltip="62.50MB"]

internal/driver/testdata/pprof.heap.flat.inuse_space.dot.focus.ignore

 digraph "unnamed" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: inuse_space\lActive filters:\l   tagfocus=30kb:\l   tagignore=1mb:2mb\lShowing nodes accounting for 36.13MB, 36.63% of 98.63MB total\lDropped 2 nodes (cum <= 4.93MB)\l"] }
+subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: inuse_space\lActive filters:\l   tagfocus=30kb:\l   tagignore=1mb:2mb\lShowing nodes accounting for 36.13MB, 36.63% of 98.63MB total\lDropped 2 nodes (cum <= 4.93MB)\l\lSee https://git.io/JfYMW for how to read the graph\l"] }
 N1 [label="line3002\n31.25MB (31.68%)\nof 32.23MB (32.67%)" id="node1" fontsize=24 shape=box tooltip="line3002 (32.23MB)" color="#b23200" fillcolor="#eddcd5"]
 NN1_0 [label = "400kB" id="NN1_0" fontsize=8 shape=box3d tooltip="31.25MB"]
 N1 -> NN1_0 [label=" 31.25MB" weight=100 tooltip="31.25MB" labeltooltip="31.25MB"]

internal/driver/testdata/pprof.heap.flat.lines.dot.focus

 digraph "unnamed" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: inuse_space\lActive filters:\l   focus=[12]00\lShowing nodes accounting for 67.38MB, 68.32% of 98.63MB total\l"] }
+subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: inuse_space\lActive filters:\l   focus=[12]00\lShowing nodes accounting for 67.38MB, 68.32% of 98.63MB total\l\lSee https://git.io/JfYMW for how to read the graph\l"] }
 N1 [label="line3000\nfile3000.src:4\n0 of 67.38MB (68.32%)" id="node1" fontsize=8 shape=box tooltip="line3000 testdata/file3000.src:4 (67.38MB)" color="#b21300" fillcolor="#edd7d5"]
 N2 [label="line2001\nfile2000.src:2\n62.50MB (63.37%)\nof 63.48MB (64.36%)" id="node2" fontsize=24 shape=box tooltip="line2001 testdata/file2000.src:2 (63.48MB)" color="#b21600" fillcolor="#edd8d5"]
 NN2_0 [label = "1.56MB" id="NN2_0" fontsize=8 shape=box3d tooltip="62.50MB"]

internal/driver/testdata/pprof.heap_alloc.flat.alloc_space.dot

 digraph "unnamed" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: alloc_space\lActive filters:\l   tagshow=[2]00\lShowing nodes accounting for 93.75MB, 95.05% of 98.63MB total\lDropped 1 node (cum <= 4.93MB)\l"] }
+subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: alloc_space\lActive filters:\l   tagshow=[2]00\lShowing nodes accounting for 93.75MB, 95.05% of 98.63MB total\lDropped 1 node (cum <= 4.93MB)\l\lSee https://git.io/JfYMW for how to read the graph\l"] }
 N1 [label="line3002\n31.25MB (31.68%)\nof 94.73MB (96.04%)" id="node1" fontsize=20 shape=box tooltip="line3002 (94.73MB)" color="#b20200" fillcolor="#edd5d5"]
 N2 [label="line3000\n0 of 98.63MB (100%)" id="node2" fontsize=8 shape=box tooltip="line3000 (98.63MB)" color="#b20000" fillcolor="#edd5d5"]
 N3 [label="line2001\n62.50MB (63.37%)\nof 63.48MB (64.36%)" id="node3" fontsize=24 shape=box tooltip="line2001 (63.48MB)" color="#b21600" fillcolor="#edd8d5"]

internal/driver/testdata/pprof.heap_alloc.flat.alloc_space.dot.focus

 digraph "unnamed" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: alloc_space\lActive filters:\l   focus=[234]00\lShowing nodes accounting for 93.75MB, 95.05% of 98.63MB total\lDropped 1 node (cum <= 4.93MB)\l"] }
+subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: alloc_space\lActive filters:\l   focus=[234]00\lShowing nodes accounting for 93.75MB, 95.05% of 98.63MB total\lDropped 1 node (cum <= 4.93MB)\l\lSee https://git.io/JfYMW for how to read the graph\l"] }
 N1 [label="line3002\n31.25MB (31.68%)\nof 94.73MB (96.04%)" id="node1" fontsize=20 shape=box tooltip="line3002 (94.73MB)" color="#b20200" fillcolor="#edd5d5"]
 NN1_0 [label = "400kB" id="NN1_0" fontsize=8 shape=box3d tooltip="31.25MB"]
 N1 -> NN1_0 [label=" 31.25MB" weight=100 tooltip="31.25MB" labeltooltip="31.25MB"]

internal/driver/testdata/pprof.heap_alloc.flat.alloc_space.dot.hide

 digraph "unnamed" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: alloc_space\lActive filters:\l   hide=line.*1?23?\lShowing nodes accounting for 93.75MB, 95.05% of 98.63MB total\lDropped 1 node (cum <= 4.93MB)\l"] }
+subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: alloc_space\lActive filters:\l   hide=line.*1?23?\lShowing nodes accounting for 93.75MB, 95.05% of 98.63MB total\lDropped 1 node (cum <= 4.93MB)\l\lSee https://git.io/JfYMW for how to read the graph\l"] }
 N1 [label="line3000\n62.50MB (63.37%)\nof 98.63MB (100%)" id="node1" fontsize=24 shape=box tooltip="line3000 (98.63MB)" color="#b20000" fillcolor="#edd5d5"]
 NN1_0 [label = "1.56MB" id="NN1_0" fontsize=8 shape=box3d tooltip="62.50MB"]
 N1 -> NN1_0 [label=" 62.50MB" weight=100 tooltip="62.50MB" labeltooltip="62.50MB"]

internal/driver/testdata/pprof.heap_sizetags.dot

 digraph "unnamed" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: inuse_space\lShowing nodes accounting for 93.75MB, 95.05% of 98.63MB total\lDropped 1 node (cum <= 4.93MB)\l"] }
+subgraph cluster_L { "Build ID: buildid" [shape=box fontsize=16 label="Build ID: buildid\lcomment\lType: inuse_space\lShowing nodes accounting for 93.75MB, 95.05% of 98.63MB total\lDropped 1 node (cum <= 4.93MB)\l\lSee https://git.io/JfYMW for how to read the graph\l"] }
 N1 [label="line3002\n31.25MB (31.68%)\nof 94.73MB (96.04%)" id="node1" fontsize=20 shape=box tooltip="line3002 (94.73MB)" color="#b20200" fillcolor="#edd5d5"]
 NN1_0 [label = "16B..64B" id="NN1_0" fontsize=8 shape=box3d tooltip="93.75MB"]
 N1 -> NN1_0 [label=" 93.75MB" weight=100 tooltip="93.75MB" labeltooltip="93.75MB"]

internal/driver/testdata/pprof.long_name_funcs.dot

 digraph "testbinary" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "File: testbinary" [shape=box fontsize=16 label="File: testbinary\lType: cpu\lDuration: 10s, Total samples = 1.11s (11.10%)\lShowing nodes accounting for 1.11s, 100% of 1.11s total\l" tooltip="testbinary"] }
+subgraph cluster_L { "File: testbinary" [shape=box fontsize=16 label="File: testbinary\lType: cpu\lDuration: 10s, Total samples = 1.11s (11.10%)\lShowing nodes accounting for 1.11s, 100% of 1.11s total\l\lSee https://git.io/JfYMW for how to read the graph\l" tooltip="testbinary"] }
 N1 [label="package1\nobject\nfunction1\n1.10s (99.10%)" id="node1" fontsize=24 shape=box tooltip="path/to/package1.object.function1 (1.10s)" color="#b20000" fillcolor="#edd5d5"]
 N2 [label="FooBar\nrun\n0.01s (0.9%)\nof 1.01s (90.99%)" id="node2" fontsize=10 shape=box tooltip="java.bar.foo.FooBar.run(java.lang.Runnable) (1.01s)" color="#b20400" fillcolor="#edd6d5"]
 N3 [label="Bar\nFoo\n0 of 1.10s (99.10%)" id="node3" fontsize=8 shape=box tooltip="(anonymous namespace)::Bar::Foo (1.10s)" color="#b20000" fillcolor="#edd5d5"]

internal/report/report.go

 				nodeCount, origCount))
 		}
 	}
+
+	// Help new users understand the graph.
+	// A new line is intentionally added here to better show this message.
+	if fullHeaders {
+		label = append(label, "\\lSee https://git.io/JfYMW for how to read the graph")
+	}
+
 	return label
 }
 

internal/report/testdata/source.dot

 digraph "unnamed" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "Duration: 10s, Total samples = 11111 " [shape=box fontsize=16 label="Duration: 10s, Total samples = 11111 \lShowing nodes accounting for 11111, 100% of 11111 total\l"] }
+subgraph cluster_L { "Duration: 10s, Total samples = 11111 " [shape=box fontsize=16 label="Duration: 10s, Total samples = 11111 \lShowing nodes accounting for 11111, 100% of 11111 total\l\lSee https://git.io/JfYMW for how to read the graph\l"] }
 N1 [label="tee\nsource2:8\n10000 (90.00%)" id="node1" fontsize=24 shape=box tooltip="tee testdata/source2:8 (10000)" color="#b20500" fillcolor="#edd6d5"]
 N2 [label="main\nsource1:2\n1 (0.009%)\nof 11111 (100%)" id="node2" fontsize=9 shape=box tooltip="main testdata/source1:2 (11111)" color="#b20000" fillcolor="#edd5d5"]
 N3 [label="tee\nsource2:2\n1000 (9.00%)\nof 11000 (99.00%)" id="node3" fontsize=14 shape=box tooltip="tee testdata/source2:2 (11000)" color="#b20000" fillcolor="#edd5d5"]