cmd/go: add support for tracing to visualize build behavior

[matloob]

I'd like to add a GODEBUG option to cmd/go to emit traces so that we can visualize build behavior. For now it will emit a runtime trace but maybe in the future we might use @ianthehat's events library. Maybe in the future it could become a flag, but for now adding it to GODEBUG should be safe in the sense that it doesn't affect the formal interface of the go command.

[matloob]

cc @bcmills @jayconrod @ianthehat @mvdan

[gopherbot]

Change https://golang.org/cl/230378 mentions this issue: cmd/go: add a trace option to GODEBUG

[josharian]

Tracing like #15736?

[matloob]

Yes, though my understanding is that --debug-actiongraph only has timing information for build actions, and doesn't cover cmd/go's behavior before actions are executed.

[gopherbot]

Change https://golang.org/cl/237683 mentions this issue: cmd/go: add a debug-trace flag to generate traces

[gopherbot]

Change https://golang.org/cl/237684 mentions this issue: cmd/go: add Context parameter to base.command.Run

[gopherbot]

Change https://golang.org/cl/238541 mentions this issue: cmd/go: propagate context into PackagesForBuild and Do for tracing

[gopherbot]

Change https://golang.org/cl/238542 mentions this issue: cmd/go/internal/trace: add function to distinguish Goroutines

[gopherbot]

Change https://golang.org/cl/238543 mentions this issue: cmd/go: add trace events for each action

[gopherbot]

Change https://golang.org/cl/239083 mentions this issue: cmd/go: mark trace flows between actions

[gopherbot]

Change https://golang.org/cl/239098 mentions this issue: cmd/trace: move viewer data structs into cmd/internal/traceviewer

[gopherbot]

Change https://golang.org/cl/239438 mentions this issue: cmd/go: process -debug-trace flag for cmd/test and cmd/vet