new metric: trace_stats, which exports the trace stats table's contents

This will be useful for bulk processing of traces, as the exported stats
can be aggregated or used when searching for sample traces for a given
property.

I've added a new top metric for this, but could also nest this within
TraceMetadata if that's preferable.

Change-Id: Icbc8827f93245efb6054ae94c06325bc3e0cf743
diff --git a/Android.bp b/Android.bp
index f71b821..6c2bee4 100644
--- a/Android.bp
+++ b/Android.bp
@@ -7780,6 +7780,7 @@
     "src/trace_processor/metrics/experimental/frame_times.sql",
     "src/trace_processor/metrics/experimental/reported_by_page.sql",
     "src/trace_processor/metrics/trace_metadata.sql",
+    "src/trace_processor/metrics/trace_stats.sql",
     "src/trace_processor/metrics/webview/webview_power_usage.sql",
   ],
   cmd: "$(location tools/gen_merged_sql_metrics.py) --cpp_out=$(out) $(in)",