Modifies statsd output for start and end times.

We include the start of when the last dump occurred and the current
timestamp. These timestamps are shared across all metrics, so
there's no advantage in duplicating these numbers across all metrics.

Also, we should use elapsed realtime instead of times based on wall
clock, which can jump around and go backwards.

Test: Test that statsd can still build and
adb shell cmd stats dump-report doesn't crash.
Change-Id: I819e5643cee75dfa3e78a58f94c9d61ededa78d7
diff --git a/cmds/statsd/src/metrics/MetricsManager.h b/cmds/statsd/src/metrics/MetricsManager.h
index 9cae70a..2b30f44 100644
--- a/cmds/statsd/src/metrics/MetricsManager.h
+++ b/cmds/statsd/src/metrics/MetricsManager.h
@@ -64,6 +64,11 @@
 
     void dumpStates(FILE* out, bool verbose);
 
+    // Returns the elapsed realtime when this metric manager last reported metrics.
+    uint64_t getLastReportTimeNs() {
+        return mLastReportTimeNs;
+    };
+
     // Config source owner can call onDumpReport() to get all the metrics collected.
     virtual void onDumpReport(android::util::ProtoOutputStream* protoOutput);
     virtual void onDumpReport(const uint64_t& dumpTimeStampNs, ConfigMetricsReport* report);
@@ -78,6 +83,8 @@
 
     bool mConfigValid = false;
 
+    uint64_t mLastReportTimeNs;
+
     // The uid log sources from StatsdConfig.
     std::vector<int32_t> mAllowedUid;