Missing ABC for Metric

[GoPavel]

Missing ABC for Metric seems strange to me.
I guess in the PR #311 and Issue #66 settle to add ABC for all interfaces.

Maybe it should look like:

class Metric(abc.ABC):
    """...
    """

    @abc.abstractmethod
    def get_handle(self, label_set: LabelSet) -> "object":
        """...
        """

[codeboten]

Thanks for catching this. Mind submitting a PR @GoPavel since you already have most of the code written up here?

[GoPavel]

Thanks for catching this. Mind submitting a PR @GoPavel since you already have most of the code written up here?

Ok. Could you review it?