Occasionally this test would flake. The flakes were fixed by:
1. Stopping the service and retrying to check on metrics. This way we
also include the active_streams going to 0 in the metric calls.
2. Using a reference to the global Metrics. This way when other tests
have background goroutines that are still shutting down, they won't
emit metrics to the metric instance with the fake Sink. The stats
test can patch the local reference to the global, so the existing
statHandlers will continue to emit to the global, but the stats
test will send all metrics to the replacement.
