Metrics are added to a materialized protobuf soong_build_metrics.pb
Recorded events have scoped names and recorded runtimes.
For example, mixedbuilds includes the following events:
mixedbuilds.prepare
mixedbuilds.analyze.write_files
mixedbuild (this is a total of all mixedbuild events)
Test: Manually verified materialized protos for mixedbuild, bp2build,
and legacy build.
Change-Id: I2e72d4502c0373bee0279bb1320b2e4dd586129c
715b08fd82