service: Add GetTraceStats() to consumer endpoint

The new API allows a consumer to obtain trace and buffer stats about the
current tracing session separately from the actual trace data. Consumers
may use this to show stats such as buffer utilization in a UI.

Also adds a few new stats to TraceBuffer.

Change-Id: I929955b2854e5b84b6d1c0ca82e750c47df77ae1
diff --git a/test/test_helper.h b/test/test_helper.h
index a5a1301..f2fe7dd 100644
--- a/test/test_helper.h
+++ b/test/test_helper.h
@@ -43,6 +43,7 @@
   void OnTraceData(std::vector<TracePacket> packets, bool has_more) override;
   void OnDetach(bool) override;
   void OnAttach(bool, const TraceConfig&) override;
+  void OnTraceStats(bool, const TraceStats&) override;
 
   void StartServiceIfRequired();
   FakeProducer* ConnectFakeProducer();