Add option to track BEP metrics in benchmarks

[meisterT]

Description of the problem / feature request:

The build event protocol exposes a bunch of good metrics about the invocation such as number of packages loaded, targets configured, actions created and executed.
It would be nice to be able to track them with bazel-bench.

Feature requests: what underlying problem are you trying to solve with this feature?

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suggested by @zhengwei143