Exposing public APIs to for implementing new MeasurementConsumer classes

[JP-MY]

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. 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

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.

Describe alternatives you've considered

No response

Additional Context

No response

Would you like to implement a fix?

Yes

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