Log bp2build_metrics .pb

Also share `Save(pb proto.Message, filepath string)`

Bug: 201539536
Test: bp2build_metrics.pb has expected content & path
Test: m nothing
Test: {bp2build,mixed_{libc,droid}}.sh
Test: CI
Change-Id: I7d8ad87fca6a4b0355010090a527f5ae67b27c88
diff --git a/ui/build/soong.go b/ui/build/soong.go
index ae9a2ce..5360342 100644
--- a/ui/build/soong.go
+++ b/ui/build/soong.go
@@ -373,6 +373,7 @@
 	soongBuildEnv.Set("BAZEL_OUTPUT_BASE", filepath.Join(config.BazelOutDir(), "output"))
 	soongBuildEnv.Set("BAZEL_WORKSPACE", absPath(ctx, "."))
 	soongBuildEnv.Set("BAZEL_METRICS_DIR", config.BazelMetricsDir())
+	soongBuildEnv.Set("LOG_DIR", config.LogsDir())
 
 	// For Soong bootstrapping tests
 	if os.Getenv("ALLOW_MISSING_DEPENDENCIES") == "true" {