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
For example, new metrics at HEAD highlight that 99 of 1379 total
cc_library_static modules are converted.
Test: m bp2build, printproto
Change-Id: I6cc4227124e9a130b75911f3e40e6585d731d00a