MetricReader
Decides when aggregated metrics are collected, and owns the MetricExporter they are handed to. Pairing the two is what makes two destinations on two intervals expressible.
Sealed because the intended extension point is MetricExporter: an exporter says only where metrics go, while a reader is coupled to AggregatingMeterProvider.collect and the provider's lifecycle. Unsealing later is a compatible change; retracting an extension point is not.