Add RBE metrics dump in Soong UI.

From aosp/1329396, the RBE metrics protobuf file is part of the
metrics uploading process. The RBE metrics protobuf file is
generated by running the bootstrap shutdown command. A new function
named DumpRBEMetrics was written in order to generate the RBE metrics
protobuf file before sending to the uploading process.

Bug: b/140638454
Test: * Unit test cases
      * Ran RBE build on my local host and verified the
        metrics protobuf file is created.
      * Ran non-RBE build after RBE build and verified that
        the previous metrics protobuf file was deleted.

Change-Id: I4b8068905cb67c4b8c2d94793917b98974fed707

cmd/soong_ui/main.go

@@ -187,6 +187,7 @@ func main() {
 		config.Parallel(), config.RemoteParallel(), config.HighmemParallel())
 
 	defer met.Dump(soongMetricsFile)
+	defer build.DumpRBEMetrics(buildCtx, config, rbeMetricsFile)
 
 	if start, ok := os.LookupEnv("TRACE_BEGIN_SOONG"); ok {
 		if !strings.HasSuffix(start, "N") {