Parse `mntr` command's content.
        `buf` is a readable file-like object

        Returns: a tuple (metrics, mode)
        if mode == 'inactive', metrics will be None

def parse_mntr(self, buf):
        """
        Parse `mntr` command's content.
        `buf` is a readable file-like object

        Returns: a tuple (metrics, mode)
        if mode == 'inactive', metrics will be None
        """
        buf.seek(0)
        first = buf.readline()  # First is version string or error
        if first == 'This ZooKeeper instance is not currently serving requests':
            return (None, 'inactive')

        metrics = []
        mode = 'inactive'

        for line in buf:
            try:
                key, value = line.split()

                if key == "zk_server_state":
                    mode = value.lower()
                    continue

                metric_name = self._normalize_metric_label(key)
                metric_type = "rate" if key in self._MNTR_RATES else "gauge"
                metric_value = int(value)
                metrics.append(ZKMetric(metric_name, metric_value, metric_type))

            except ValueError:
                self.log.warning(u"Cannot format `mntr` value. key={key}, value{value}".format(key=key, value=value))
                continue
            except Exception:
                self.log.exception(
                    u"Unexpected exception occurred while parsing `mntr` command content:\n{buf}".format(buf=buf)
                )

        return (metrics, mode)