Add performance counter metrics to build.trace.gz

Start a background goroutine at the beginning of soong_build that
captures the CPU usage, heap size, and total system memory every
second.  Propagate the values through soong_build_metrics.pb back
to soong_ui, and then into build.trace.gz.

Test: m nothing, examine build.trace.gz
Change-Id: Iad99f8f1f088f4f7f7d5f76566a38c0c4f4d0daa

cmd/soong_build/main.go

@@ -422,6 +422,7 @@ func main() {
 	metricsDir := availableEnv["LOG_DIR"]
 
 	ctx := newContext(configuration)
+	android.StartBackgroundMetrics(configuration)
 
 	var finalOutputFile string