Return a dictionary of runtime statistics.

        This dictionary will be empty when the controller has never been
        ran. When it is running or has ran previously it should have (but
        may not) have useful and/or informational keys and values when
        running is underway and/or completed.

        .. warning:: The keys in this dictionary **should** be some what
                     stable (not changing), but there existence **may**
                     change between major releases as new statistics are
                     gathered or removed so before accessing keys ensure that
                     they actually exist and handle when they do not.

        .. note:: The values in this dictionary are local to the thread
                  running call (so if multiple threads share the same retrying
                  object - either directly or indirectly) they will each have
                  there own view of statistics they have collected (in the
                  future we may provide a way to aggregate the various
                  statistics from each thread).

def statistics(self):
        """Return a dictionary of runtime statistics.

        This dictionary will be empty when the controller has never been
        ran. When it is running or has ran previously it should have (but
        may not) have useful and/or informational keys and values when
        running is underway and/or completed.

        .. warning:: The keys in this dictionary **should** be some what
                     stable (not changing), but there existence **may**
                     change between major releases as new statistics are
                     gathered or removed so before accessing keys ensure that
                     they actually exist and handle when they do not.

        .. note:: The values in this dictionary are local to the thread
                  running call (so if multiple threads share the same retrying
                  object - either directly or indirectly) they will each have
                  there own view of statistics they have collected (in the
                  future we may provide a way to aggregate the various
                  statistics from each thread).
        """
        try:
            return self._local.statistics
        except AttributeError:
            self._local.statistics = {}
            return self._local.statistics