OM 1.0-2.0: MetricSet is not defined

[bwplotka]

I can't find MetricSet definition, yet is used across the spec. Let's define it.

[bwplotka]

Oh it is, but after we use it... ):

https://github.com/prometheus/docs/blob/main/docs/specs/om/open_metrics_spec.md#metricset

Maybe it's fine.