Bug 1372668 - [mozlog] Limit the size of data logged from error message when suite_start called multiple times, r=jgraham

We've seen a couple of enormous log files (200MB+). This largely happened due to a bug in the test harness that resulted
in suite_start being called over and over again. In each of these instances, mozlog logged something like:
log.error("Suite start called multiple times: {}".format(data))

The problem is that 'data' contained every single test id in the suite, which was *a lot*. Dumping all test ids in that
error message is not useful for debugging. This patch limits the size of the 'suite_start' data in the error message to
100 characters.

MozReview-Commit-ID: GnPizNrZ2QJ

--HG--
extra : rebase_source : 985d484544da9ea4cce445ce406fe085f86f112b

testing/mozbase/mozlog/mozlog/structuredlog.py

@@ -254,8 +254,9 @@ class StructuredLogger(object):
     def _ensure_suite_state(self, action, data):
         if action == 'suite_start':
             if self._state.suite_started:
+                # limit data to reduce unnecessary log bloat
                 self.error("Got second suite_start message before suite_end. " +
-                           "Logged with data: {}".format(json.dumps(data)))
+                           "Logged with data: {}".format(json.dumps(data)[:100]))
                 return False
             self._state.suite_started = True
         elif action == 'suite_end':