service: Add stats for discarded chunks + patches

Adds some stats for chunks + patches that are discarded by the service
before passing them on to a TraceBuffer.

Also updates error message logging levels.

Bug: chromium:919187
Change-Id: I88d4996c0cfc2fb4c41ac71f805388092aab9634
diff --git a/src/tracing/core/tracing_service_impl.h b/src/tracing/core/tracing_service_impl.h
index d29535a..a7f06bc 100644
--- a/src/tracing/core/tracing_service_impl.h
+++ b/src/tracing/core/tracing_service_impl.h
@@ -438,6 +438,10 @@
   uint8_t sync_marker_packet_[32];  // Lazily initialized.
   size_t sync_marker_packet_size_ = 0;
 
+  // Stats.
+  uint64_t chunks_discarded_ = 0;
+  uint64_t patches_discarded_ = 0;
+
   PERFETTO_THREAD_CHECKER(thread_checker_)
 
   base::WeakPtrFactory<TracingServiceImpl>