Implement "record" for MeasureMetric

[lzchen]

Implementation of record for MeasureMetric in the SDK is not done.
There is still discussion about how we handle aggregation and configurations as well.

Part of [#155]
Follows [#160]

[mauriciovasquezbernal]

@lzchen https://github.com/open-telemetry/opentelemetry-python/pull/422/files#diff-ceb6c75131a7c968ee965da2a10fa5d2R51 implemented the MinMaxSumCount aggregator for Measure instruments, does it suffice or there is something missing for closing this issue?

[lzchen]

I think this issue needs to be better scoped, it was created a long time ago when record was not implemented yet. Ideally for this to be "complete", all aggregations for measure should be finished but we can probably track those as separate issues. I'm fine with closing this one.

[lzchen]

I think this issue can be closed. The default aggregation for measure is sufficient for the beta release, as the other aggregation types have not yet made it to the specs. I will create another issue for them once they are finalized in the specs.

@c24t @toumorokoshi