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

shared/Android.bp

@@ -9,11 +9,13 @@ bootstrap_go_package {
         "env.go",
         "paths.go",
         "debug.go",
+        "proto.go",
     ],
     testSrcs: [
         "paths_test.go",
     ],
     deps: [
         "soong-bazel",
+        "golang-protobuf-proto",
     ],
 }

shared/proto.go

@@ -0,0 +1,41 @@
+// Copyright 2021 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package shared
+
+import (
+	"io/ioutil"
+	"os"
+
+	"google.golang.org/protobuf/proto"
+)
+
+// Save takes a protobuf message, marshals to an array of bytes
+// and is then saved to a file.
+func Save(pb proto.Message, filepath string) (err error) {
+	data, err := proto.Marshal(pb)
+	if err != nil {
+		return err
+	}
+	tempFilepath := filepath + ".tmp"
+	if err := ioutil.WriteFile(tempFilepath, []byte(data), 0644 /* rw-r--r-- */); err != nil {
+		return err
+	}
+
+	if err := os.Rename(tempFilepath, filepath); err != nil {
+		return err
+	}
+
+	return nil
+}