Add metrics collection to soong_build

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