Internal processorhelper metrics are not reported until they have any non-zero values

[dmitryax]

After #10693, the processor helper only reports metrics if they got any values other than 0. This is a significant change in the behavior compared to previous versions when we had all of them reported at once. We should document this in the changelog.

As a user, I would always prefer seeing 0 data points reported for any data sent through a processor for otelcol_processor_dropped_spans and otelcol_processor_refused_spans metrics. Happy to hear what other folks think though.