'Sets value of a given gauge metric.
Args:
varname: Metric name.
value: New metric value.
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.'

@utils.Synchronized
def SetGaugeValue(self, varname, value, fields=None):

    self._metrics[varname].Set(value, fields)
