Proposal for metric proto refactor

Update metric descriptor to contain information on the data qualities.
This includes:

 - The meaning of the time interval measurements were made over is now
   described by the `interval` value.
 - The monotonic nature of the data is captured if known.
 - If the data values are restricted to a subset of numbers.

The `SummaryDataPoint` and `HistogramDataPoint` were merged into a
unified `Distribution` message that contains statistics about the
observed population of values.

Adds support for multiple histogram bucket definitions and adds a linear
bucket definition message.

Includes an explicit minimum and maximum for a `Distribution` (open-telemetry#122).

Removes the exemplar code (open-telemetry#81). This can be added back in as a more
generalized case now given the new `Data` structure of the Metrics. But,
it has been left for a separate PR.

Unifies the common parts of the previous `*DataPoints` into a unified
`Data` message.

This is not complete. It likely needs better names and it certainly
needs comments.

Related to open-telemetry#125
Fixes to open-telemetry#81