[autotest] Fix fields provided to rpc_flight_recorder metric

BUG=chromium:715386
TEST=None

Change-Id: If619e86f4d02d3ee5936cea5a5a9b46dededabd1
Reviewed-on: https://chromium-review.googlesource.com/522866
Commit-Ready: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Chris Ching <chingcodes@chromium.org>
diff --git a/site_utils/rpc_flight_recorder.py b/site_utils/rpc_flight_recorder.py
index 25e8ca8..8e16633 100755
--- a/site_utils/rpc_flight_recorder.py
+++ b/site_utils/rpc_flight_recorder.py
@@ -95,7 +95,7 @@
         metric_fields['failure_reason'] = ''
 
         with metrics.SecondsTimer(METRIC_RPC_CALL_DURATIONS,
-                fields=dict(self._metric_fields)) as f:
+                fields=dict(metric_fields)) as f:
             try:
                 result = self._afe.run(cmd)
                 f['success'] = True