'Gets a histogram based on a key, creates a new one if it does not exist.
:param key: name of the metric
:type key: C{str}
:return: L{Histogram}'

def histogram(self, key):

    if (key not in self._histograms):
        self._histograms[key] = Histogram(clock=self._clock)
    return self._histograms[key]
