Duration shouldn't be negative. That's weird.

973 2017-02-18 19:49:55.620 E: post_error(Failed to call hook get_state(): Monotonically increasing metric "swarming/bots/hooks/durations" was given value "-28793.3863618", which is not greater than or equal to "None".
Traceback (most recent call last):
  File "swarming_bot.1.zip/bot_code/bot_main.py", line 213, in _call_hook_safe
    return _call_hook(chained, botobj, name, *args)
  File "swarming_bot.1.zip/bot_code/bot_main.py", line 162, in hook
    return fn(*args, **kwargs)
  File "swarming_bot.1.zip/bot_code/bot_main.py", line 187, in _call_hook
    return hook(botobj, *args)
  File "swarming_bot.1.zip/config/bot_config.py", line 125, in hook
    hooks_durations.add(time.time() - start, fields=fields)
  File "swarming_bot.1.zip/infra_libs/ts_mon/common/metrics.py", line 589, in add
    self._incr(fields, target_fields, value, modify_fn=modify_fn)
  File "swarming_bot.1.zip/infra_libs/ts_mon/common/metrics.py", line 321, in _incr
    target_fields, delta, modify_fn=modify_fn)
  File "swarming_bot.1.zip/infra_libs/ts_mon/common/metric_store.py", line 281, in incr
    raise errors.MonitoringDecreasingValueError(name, None, delta)
MonitoringDecreasingValueError: Monotonically increasing metric "swarming/bots/hooks/durations" was given value "-28793.3863618", which is not greater than or equal to "None".