'Attached callback to the gauge metric.
Args:
varname: Metric name.
callback: Callback function. This function is called everytime
this metric\'s value (with corresponding fields values) is
queried. Called without any arguments.
fields: Values for this metric\'s fields. For example, if metric
was registered with fields like
[("flow_type", int), ("user_type", int)], then for that metric
fields argument may look like: fields=[10, 12]. Should be None
if the metric was registered without any fields. If not None,
callback will only be called when caller provides same fields
values.'

@utils.Synchronized
def SetGaugeCallback(self, varname, callback, fields=None):

    self._metrics[varname].SetCallback(callback, fields)
