Add information about the action cache statistics in the Build Event Protocol

[crydell-ericsson]

Description of the feature request:

We think it would be good to include information about the action cache statistics somewhere in the Build Event Protocol. We are mainly interested in seeing information about the cache hits, similar to how information about runner counts is currently included, but the action cache statistics also contain other data that might be interesting to transfer over the BEP.

The simplest approach that would satisfy our use case could be to add a new entry to the RunnerCounts message that tracks the number of action cache hits, though this may be pragmatically dubious since the runners are functionally distinct from the action cache.

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

Actions that are resolved via the action cache are not counted among the runners listed in the RunnerCounts message of the BEP, and there currently does not seem to be any other way of explicitly getting information about the action cache statistics. Including this information in the BEP would provide information that could be useful in improving the structure of builds.

Which operating system are you running Bazel on?

Red Hat Enterprise Linux 7.9

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 master; 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?

No response

[tbaing]

We're interested in this as well. We'd be interested in knowing not only how many cache hits eliminated the need for an action, but specifically which ones they were so we can aggregate statistics about per-action cache hit rates over many builds.

[github-actions]

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