feat(config): add pull metric reader support to declarative config

[MikeGoldsmith]

The declarative config MeterProvider implementation currently raises an error when pull metric readers are specified (e.g. Prometheus), with a message directing users to configure pull readers via the SDK API directly.

The OTel declarative config spec lists pull metric readers (PullMetricExporter) as a supported plugin component type. This issue tracks implementing proper support for pull metric readers in _meter_provider.py.

Related: #3631

[ravitheja4531-cell]

Hi, I'd like to work on this. My understanding is the fix involves implementing PullMetricExporter plugin resolution in _meter_provider.py, aligned with the OTel declarative config spec. I'll look at #3631 for context on the existing architecture. Let me know if there's a preferred approach or if this is already being worked on.