Remove duplicate metricset serialization logic

We currently have the logic for metricset serialization in three places:
* Internal metrics
* Micrometer metrics
* OpenTelemetry metrics

With #2968 we added the `MetricSetSerializer` class, which is used for the OpenTelemetry metricset serialization.
This class is generic enough that we should be able to extract a superclass which delegates just the `labels` serialization to its subclasses. We can then use this base class for all places where we serialize metricsets.