Add otelcol.processor.metricstransformprocessor component

[Thiryn]

Request

Add the OTEL module metricstransformprocessor.

Use case

The otelcol.processor.transform is using the generic OTTL based transform processor.
This processor is in alpha and does not currently have all the transform features from the existing metricstransformprocessor.

The transformprocessor relies on the OTTL grammar, which is currently limited (see the available OTTL functions list).

The one feature I currently need is to aggregate metrics by label, there is an open issue (recently active) to add it to OTTL but is not implemented at the time.

[github-actions]

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