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/metrics/Android.bp b/ui/metrics/Android.bp
index 96f6389..3ba3907 100644
--- a/ui/metrics/Android.bp
+++ b/ui/metrics/Android.bp
@@ -25,6 +25,7 @@
         "soong-ui-metrics_proto",
         "soong-ui-bp2build_metrics_proto",
         "soong-ui-tracer",
+        "soong-shared",
     ],
     srcs: [
         "metrics.go",