feat(config): add metric producer support to declarative config

[MikeGoldsmith]

The declarative config MeterProvider implementation does not currently handle metric producers (MetricProducer). This component type is listed in the OTel declarative config spec as a supported plugin component but is not implemented in _meter_provider.py.

Related: #3631

[ravitheja4531-cell]

Hi, I'd like to work on this if it's available.

My plan:

• Follow the entry-point loading pattern being established in feat(config): generic resource detector loading in declarative config #5053 to instantiate each MetricProducer from the config
• Pass the resolved list as metric_producers=[...] to each MetricReader constructor in _meter_provider.py
• Add tests mirroring the existing patterns in tests/_configuration/test_meter_provider.py

One question before I start: should metric_producers be loaded exclusively via entry points (consistent with #5053), or is there also a need to handle known built-in producers (e.g. Prometheus) with a direct import path similar to how OTLP exporters are handled?

Happy to wait until #5053 lands if the entry-point infrastructure isn't ready yet.