Counter reset that sets every value to zero

I have a counter metric with a single label. It is initialized to zero for all possible values of the label, as is a best practice for counter metrics. This happens immediately after the metric is defined.

In my unit testing framework, I reset all metrics after each test, to make it easy to test the metrics themselves. However, this erases my initialization of everything to zero. That means that if I try to check that the value of my metric at a label is zero, it fails, because the metric actually has no value at that label.

It would be nice to have a version of reset() which sets counter metrics to zero. I guess it could be called "zeroOut", or something like that, and then there would be a "zeroOutMetrics" like "resetMetrics".

I suppose this function should also work on other metric types, for consistency.

I would be happy to open a pull request if you believe it is a good idea.

Start by reading the existing reset() and resetMetrics() implementations and the tests covering metric reset behavior. Clarify the API and semantics for counters and other metric types; done should preserve initialized label values at zero after the new reset operation, with tests covering the supported metrics.