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
diff --git a/ui/build/config.go b/ui/build/config.go
index 1dd948c..077a4d1 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -35,10 +35,10 @@
 )
 
 const (
-	envConfigDir  = "vendor/google/tools/soong_config"
-	jsonSuffix    = "json"
+	envConfigDir = "vendor/google/tools/soong_config"
+	jsonSuffix   = "json"
 
-	configFetcher = "vendor/google/tools/soong/expconfigfetcher"
+	configFetcher         = "vendor/google/tools/soong/expconfigfetcher"
 	envConfigFetchTimeout = 10 * time.Second
 )
 
@@ -62,6 +62,7 @@
 	jsonModuleGraph bool
 	bp2build        bool
 	queryview       bool
+	reportMkMetrics bool // Collect and report mk2bp migration progress metrics.
 	soongDocs       bool
 	skipConfig      bool
 	skipKati        bool
@@ -155,7 +156,7 @@
 	}
 
 	configExists := false
-	outConfigFilePath := filepath.Join(config.OutDir(), envConfigName + jsonSuffix)
+	outConfigFilePath := filepath.Join(config.OutDir(), envConfigName+jsonSuffix)
 	if _, err := os.Stat(outConfigFilePath); err == nil {
 		configExists = true
 	}
@@ -711,6 +712,8 @@
 			c.skipConfig = true
 		} else if arg == "--skip-soong-tests" {
 			c.skipSoongTests = true
+		} else if arg == "--mk-metrics" {
+			c.reportMkMetrics = true
 		} else if len(arg) > 0 && arg[0] == '-' {
 			parseArgNum := func(def int) int {
 				if len(arg) > 2 {
@@ -1381,6 +1384,11 @@
 	return filepath.Join(c.LogsDir(), "bazel_metrics")
 }
 
+// MkFileMetrics returns the file path for make-related metrics.
+func (c *configImpl) MkMetrics() string {
+	return filepath.Join(c.LogsDir(), "mk_metrics.pb")
+}
+
 func (c *configImpl) SetEmptyNinjaFile(v bool) {
 	c.emptyNinjaFile = v
 }