    def inc(self, amount: float = 1, exemplar: Optional[Dict[str, str]] = None) -> None:
        """Increment counter by the given amount."""
        self._raise_if_not_observable()
        if amount < 0:
            raise ValueError('Counters can only be incremented by non-negative amounts.')
        self._value.inc(amount)
        if exemplar:
            _validate_exemplar(exemplar)
            self._value.set_exemplar(Exemplar(exemplar, amount, time.time()))
Increment counter by the given amount.
prometheus_client/metrics.py