Metrics: Refine Observer aggregation options

[jmacd]

We are familiar with three common ways to aggregate events for observer instruments, but we have avoided giving users the ability to configure aggregations directly. This suggests we may want to refine Observer into three distinct semantic kinds, based on the desired aggregation behavior.

There are observer instruments that report cumulative values. These are Counter-like.
There are observer instruments that report delta values. These are also Counter-like.
There are observer instruments that report measurements. These are Measure-like.

In each of these cases, the questions are (1) how to combine values for different label sets, (2) how to export in systems like Prometheus.

See the comment: #430 (comment)

[jmacd]

This will close when OTEP 98 is approved and merged into the specification.
Please review: open-telemetry/oteps#98