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/bp2build/metrics.go b/bp2build/metrics.go
index 8a0b1c9..04fac44 100644
--- a/bp2build/metrics.go
+++ b/bp2build/metrics.go
@@ -43,6 +43,8 @@
 
 	// Counts of total modules by module type.
 	totalModuleTypeCount map[string]uint64
+
+	Events []*bp2build_metrics_proto.Event
 }
 
 // Serialize returns the protoized version of CodegenMetrics: bp2build_metrics_proto.Bp2BuildMetrics
@@ -55,6 +57,7 @@
 		ConvertedModules:         metrics.convertedModules,
 		ConvertedModuleTypeCount: metrics.convertedModuleTypeCount,
 		TotalModuleTypeCount:     metrics.totalModuleTypeCount,
+		Events:                   metrics.Events,
 	}
 }