Sandboxing time breakdown in tracing profile needed

[ob]

As discussed at BazelCon, debugging the sandbox performance problem on macOS would be easier if there was an option to add to the trace profile a break down, per action, of the time it takes to set up the sandbox, the time it takes to execute the action’s command, and the time it takes to tear down the sandbox.

[ob]

@meisterT, I just got around to trying this... but I'm not sure what I should be seeing. I ran bazel as

bazel build --genrule_strategy=sandboxed --spawn_strategy=sandboxed --config Debug --experimental_generate_json_trace_profile --experimental_profile_cpu_usage --profile trace-profile.json //:App

And got this:

I was expecting something more like this:

[meisterT]

It's there, it's just super small :-)

See my screenshots:

Same action, zoomed in:

So the super thin blue bars are the sandboxing setup.

[meisterT]

I just realized that this just the Java object creation. I'll create a follow-up commit...