'Increments a counter metric by a given delta.
Args:
varname: Metric name.
delta: Delta by which the metric should be incremented.
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.
Raises:
ValueError: If delta < 0.'

@utils.Synchronized
def IncrementCounter(self, varname, delta=1, fields=None):

    self._metrics[varname].Increment(delta, fields)
