Remove duplicate metricset serialization logic

[JonasKunz]

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.