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/bp2build/build_conversion.go b/bp2build/build_conversion.go
index eb60cd1..54b59af 100644
--- a/bp2build/build_conversion.go
+++ b/bp2build/build_conversion.go
@@ -259,7 +259,7 @@
 
 	// Simple metrics tracking for bp2build
 	metrics := CodegenMetrics{
-		ruleClassCount: make(map[string]int),
+		ruleClassCount: make(map[string]uint64),
 	}
 
 	dirs := make(map[string]bool)