Report mkfile metrics with --mk-metrics

Combined with roboleaf CI changes to pass this flag and subsequent
pipeline changes, this will allow basic mkfile metrics to be
dashboarded, which will help track mk2bp progress.

Bug: 217764943
Test: manual; verified mk_metrics.pb is generated iff --mk-metrics is
specified, used printproto on result

Change-Id: I348994b4d7e3479d0dc04de0276b94a702aac95b

ui/metrics/metrics.go

@@ -41,6 +41,7 @@ import (
 	"google.golang.org/protobuf/proto"
 
 	soong_metrics_proto "android/soong/ui/metrics/metrics_proto"
+	mk_metrics_proto "android/soong/ui/metrics/mk_metrics_proto"
 )
 
 const (
@@ -62,14 +63,22 @@ const (
 	Total = "total"
 )
 
-// Metrics is a struct that stores collected metrics during the course
-// of a build which later is dumped to a MetricsBase protobuf file.
-// See ui/metrics/metrics_proto/metrics.proto for further details
-// on what information is collected.
+// Metrics is a struct that stores collected metrics during the course of a
+// build. It is later dumped to protobuf files. See underlying metrics protos
+// for further details on what information is collected.
 type Metrics struct {
-	// The protobuf message that is later written to the file.
+	// Protobuf containing various top-level build metrics. These include:
+	// 1. Build identifiers (ex: branch ID, requested product, hostname,
+	//    originating command)
+	// 2. Per-subprocess top-level metrics (ex: ninja process IO and runtime).
+	//    Note that, since these metrics are reported by soong_ui, there is little
+	//    insight that can be provided into performance breakdowns of individual
+	//    subprocesses.
 	metrics soong_metrics_proto.MetricsBase
 
+	// Protobuf containing metrics pertaining to number of makefiles in a build.
+	mkMetrics mk_metrics_proto.MkMetrics
+
 	// A list of pending build events.
 	EventTracer *EventTracer
 }
@@ -78,11 +87,24 @@ type Metrics struct {
 func New() (metrics *Metrics) {
 	m := &Metrics{
 		metrics:     soong_metrics_proto.MetricsBase{},
+		mkMetrics:   mk_metrics_proto.MkMetrics{},
 		EventTracer: &EventTracer{},
 	}
 	return m
 }
 
+func (m *Metrics) SetTotalMakefiles(total int) {
+	m.mkMetrics.TotalMakefiles = uint32(total)
+}
+
+func (m *Metrics) SetToplevelMakefiles(total int) {
+	m.mkMetrics.ToplevelMakefiles = uint32(total)
+}
+
+func (m *Metrics) DumpMkMetrics(outPath string) {
+	shared.Save(&m.mkMetrics, outPath)
+}
+
 // SetTimeMetrics stores performance information from an executed block of
 // code.
 func (m *Metrics) SetTimeMetrics(perf soong_metrics_proto.PerfInfo) {