Exposing public APIs to for addition and removal of metric readers at run-time #4818

Is your feature request related to a problem?

We need to re-configure opentelemetry processors at run-time to control where telemetry signals are exported to, but the SDK does not allow resetting of MeterProvider, TracerProvider or LoggerProvider, or any APIs to add or remove processors / readers. We have worked around this limitation by using SynchronousMulti<Span/LogRecord>Processors and adding / removing other processors to them (removing processors is not supported in the current SDK but is easy to implement by following existing conventions and extending public APIs), however there doesn't seem to be any equivalent for metrics, so we have no way of re-configuring which metric readers are used during run-time.

Describe the solution you'd like

Following discussions with the opentelemetry team, it was decided that adding public APIs to the metric reader is perhaps the best solution as it limits the scope of exposed public APIs.

Describe alternatives you've considered

The main thing preventing us from implementing a SynchronousMultiMeasurementConsumer is that there is only one MeasurementConsumer (SynchronousMeasurementConsumer) which is defined in _internal packages with no public API available. MeterProvider also takes no arguments about which MeasurementConsumer it should use and assumes there is only ever one implementation so it creates an instance of SynchronousMeasurementConsumer in a private property (self._measurement_consumer). Would it be possible to expose these options externally? We have proof of concept code which works by using these private APIs but we thought it'd be best if contribute this back to the project because it helps make the SDK more extensible.

Additional Context

No response

Would you like to implement a fix?

Yes

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