Allow providing a "MetricProcessor", similar to a SpanProcessor

Is your feature request related to a problem?

I'm maintaining OpenLLMetry and we have an SDK that wraps the instrumentations and enriches them with different properties. For traces, we use the SpanProcessor to tap into on_start and add attributes to relevant spans. We want to do something similar for metrics (add some attributes), so ideally, we'd love to have a way to do that before a metric is added / aggregated on the OpenTelemetry SDK level.

Describe the solution you'd like

I'd imagine something on the level of the consume_measurement - potentially allowing to specify a callback / attribute enrichments, or overriding the consumer.

Describe alternatives you've considered

I couldn't find any other way to add attributes to a metric after it's recorded.

Additional Context

No response

Would you like to implement a fix?

Yes

Start by comparing the existing SpanProcessor behavior with the metric SDK path around consume_measurement. Determine where a processor, callback, or attribute-enrichment hook could run before measurements are added or aggregated. Done means the SDK exposes a documented way to enrich metric attributes before aggregation, with coverage for the new behavior.