Add MeasurementConsumer to metrics SDK

[aabmass]

See design doc section on MeasurementConsumer. For this issue:

• Create ABC MeasurementConsumer interface and an empty implementation of DefaultMeasurementConsumer (implementation is tracked in Implement DefaultMeasurementConsumer #2328).
• Add a member measurement_consumer: MeasurementConsumer on MeterProvider
• Wire instruments to call into its methods (e.g. Counter.add() should pass measurement to measurement_consumer.consume_measurement())