sysmon: Make a copy of dict before modifying it.

I was wondering why the age metric does not exist yet.  Now I know.
And knowing is half the battle.

BUG=None
TEST=None

Change-Id: I9d75ad204c970bc5d314acf724d89f55fc1a2607
Reviewed-on: https://chromium-review.googlesource.com/707678
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
diff --git a/scripts/sysmon/puppet_metrics.py b/scripts/sysmon/puppet_metrics.py
index 1d864e2..f55efb8 100644
--- a/scripts/sysmon/puppet_metrics.py
+++ b/scripts/sysmon/puppet_metrics.py
@@ -87,7 +87,7 @@
   @property
   def times(self):
     """Return mapping of time information."""
-    times = self._data.get('time', {})
+    times = self._data.get('time', {}).copy()
     times.pop('last_run', None)
     total = times.pop('total', None)
     times['other'] = max(0, total - sum(times.itervalues()))