sync: Log repo sync state events as 'data' events.

git_trace2_event_log.py:
+ Added LogDataConfigEvents method to log 'data' events.
  Sync's current_sync_state and previous_sync_state are logged
  as 'data' events in the current log.

  It logs are key/value in the |config| argument. Each key is
  prefixed with |prefix| argument.

  The following are sample events that are logged during repo sync.

   {"event":"data",
   "sid":"repo-20210914T181545Z-P000330c0/repo-20210914T181545Z-P000330c0",
   "thread":"MainThread",
   "time":"2021-09-14T18:16:19.935846Z",
   "key":"previous_sync_state/repo.syncstate.main.synctime",
   "value":"2021-09-14T17:27:11.573717Z"}

   {"event":"data",
   "sid":"repo-20210914T181545Z-P000330c0/repo-20210914T181545Z-P000330c0",
   "thread":"MainThread",
   "time":"2021-09-14T18:16:19.955546Z",
   "key":"current_sync_state/repo.syncstate.main.synctime",
   "value":"2021-09-14T18:16:19.935979Z"}

tests/test_git_trace2_event_log.py:
+ Added unit tests

sync.py:
+ Changed logging calls to LogDataConfigEvents.

Tested:
$ ./run_tests

Tested it by running the following command multiple times.
$ repo_dev sync -j 20
  repo sync has finished successfully

  Verified config data is looged in trace2 event logs.

Bug: [google internal] b/199758376
Change-Id: I75fd830e90c1811ec28510538c99a2632b104e85
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/317823
Reviewed-by: Josh Steadmon <steadmon@google.com>
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>

git_trace2_event_log.py

@@ -167,6 +167,22 @@ class EventLog(object):
     repo_config = {k: v for k, v in config.items() if k.startswith('repo.')}
     self.LogConfigEvents(repo_config, 'def_param')
 
+  def LogDataConfigEvents(self, config, prefix):
+    """Append a 'data' event for each config key/value in |config| to the current log.
+
+    For each keyX and valueX of the config, "key" field of the event is '|prefix|/keyX'
+    and the "value" of the "key" field is valueX.
+
+    Args:
+      config: Configuration dictionary.
+      prefix: Prefix for each key that is logged.
+    """
+    for key, value in config.items():
+      event = self._CreateEventDict('data')
+      event['key'] = f'{prefix}/{key}'
+      event['value'] = value
+      self._log.append(event)
+
   def ErrorEvent(self, msg, fmt):
     """Append a 'error' event to the current log."""
     error_event = self._CreateEventDict('error')