Update MetricReader.collect to call into the MeterProvider's measurement consumer

[aabmass]

Blocked by #2326

Collecting from the SDK is done through MeasurementConsumer.collect(reader) method. When MetricReaders are added to a MeterProvider, they need to be given a handle on this method so they can perform collection. Update MetricReader.collect() to be a final implementation which calls this method.

For one possible implementation, see prototype code

[srikanthccv]

With latest merge of metric reader PR, the work left is updating the meter provider initialisation to set callback here?

[aabmass]

It looks like this is actually done @lonewolf3739?

[srikanthccv]

I don't think it is correct. There is no _register_measurement_consumer on metric reader and self there refers to meter provider. It should be updated to this unless I am missing something.

        for metric_reader in self._sdk_config.metric_readers:
            metric_reader._set_collect_callback(self._measurement_consumer.collect)

[codeboten]

Yup, can confirm that still needs updating @lonewolf3739 👍