Add option to track BEP metrics in benchmarks

**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?**

Get insights into project growth.

suggested by @zhengwei143

Start by reviewing bazel-bench's existing benchmark metric collection and how the tool represents results. Read the Build Event Protocol metrics described in the issue and determine which metrics and configuration option are in scope. Done means benchmarks can optionally record the selected BEP invocation metrics and project growth can be compared.

Written by the indexing model from the issue text.