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/shared/Android.bp b/shared/Android.bp
index deb17f8..3c84f55 100644
--- a/shared/Android.bp
+++ b/shared/Android.bp
@@ -9,11 +9,13 @@
         "env.go",
         "paths.go",
         "debug.go",
+        "proto.go",
     ],
     testSrcs: [
         "paths_test.go",
     ],
     deps: [
         "soong-bazel",
+        "golang-protobuf-proto",
     ],
 }