Please review open-telemetry/oteps#4

The four RFCs are summarized as:

1. Propose a new requirement that all metrics/stats support pre-defined labels
2. Propose a concrete implementation of the new requirement
3. Propose that stats.Record is not a requirement
4. Propose how to let the programmer suggest Metric aggregations at the point of definition.

These are intended as stand-alone issues, e.g., if you accept 001 and not 002 we can discuss other approaches. If 001 and 002 are accepted, 003 is an option. I see 004 as the key to making 001 and 002 work, but am open to other ideas.