include symlink metrics in bp2build_metrics.pb

Bug: b/256212479
Test: Prior to thi CL bp2build.symlink_forest event was missing in bp2build_metrics.pb after a clean mixed build
Change-Id: I53bfc4114a383c0d1f9c4c7945e7d4c69bc50b0c
diff --git a/bp2build/bp2build.go b/bp2build/bp2build.go
index 0e3d2a5..a75a84e 100644
--- a/bp2build/bp2build.go
+++ b/bp2build/bp2build.go
@@ -26,7 +26,7 @@
 // Codegen is the backend of bp2build. The code generator is responsible for
 // writing .bzl files that are equivalent to Android.bp files that are capable
 // of being built with Bazel.
-func Codegen(ctx *CodegenContext) CodegenMetrics {
+func Codegen(ctx *CodegenContext) *CodegenMetrics {
 	// This directory stores BUILD files that could be eventually checked-in.
 	bp2buildDir := android.PathForOutput(ctx, "bp2build")
 	if err := android.RemoveAllOutputDir(bp2buildDir); err != nil {
@@ -48,7 +48,7 @@
 	soongInjectionDir := android.PathForOutput(ctx, bazel.SoongInjectionDirName)
 	writeFiles(ctx, soongInjectionDir, CreateSoongInjectionFiles(ctx.Config(), res.metrics))
 
-	return res.metrics
+	return &res.metrics
 }
 
 // Get the output directory and create it if it doesn't exist.