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
diff --git a/ui/build/soong.go b/ui/build/soong.go
index 1b993e1..c7f22f9 100644
--- a/ui/build/soong.go
+++ b/ui/build/soong.go
@@ -537,7 +537,7 @@
 }
 
 func loadSoongBuildMetrics(ctx Context, config Config) *soong_metrics_proto.SoongBuildMetrics {
-	soongBuildMetricsFile := filepath.Join(config.OutDir(), "soong", "soong_build_metrics.pb")
+	soongBuildMetricsFile := filepath.Join(config.LogsDir(), "soong_build_metrics.pb")
 	buf, err := ioutil.ReadFile(soongBuildMetricsFile)
 	if err != nil {
 		ctx.Fatalf("Failed to load %s: %s", soongBuildMetricsFile, err)