Surface toolchain and platform information through Build Event

[sluongng]

Description of the feature request:

Toolchains and Platforms and the automatic resolution system could get quite confusing for users who are new to RBE.

It would be nice if we could surface the toolchains, platforms (host, exec, target), related constraint_{setting,value} being used, and the auto-resolution result through Build Event Stream. This way, Build Event Service providers could surface this information to end users in an easier-to-consume format. Users could then share the events with relevant build teams to help debug common issues.

Which category does this issue belong to?

Core

What underlying problem are you trying to solve with this feature?

Platforms and Toolchains matrixes are hard to learn and debug.

Which operating system are you running Bazel on?

No response

What is the output of bazel info release?

No response

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

I think we could start with an event proto message for automatic toolchain resolution.

Ultimately, I think detailed information (Providers) from CC, Java, and/or any Starlark toolchains would be super useful to have as dedicated events as well.