Add temporality to MetricDescriptor (#140)

* Add temporality and refinements to MetricDescriptor

* Update type

* Decouple histogram from temporality

* Feedback

* Feedback

Fix grammar.

* Feedback

Update comments.

* Update opentelemetry/proto/metrics/v1/metrics.proto

Co-Authored-By: Tigran Najaryan <4194920+tigrannajaryan@users.noreply.github.com>

* Feedback

Run make.
Remove errant space.

* Feedback

Add info about INSTANTANEOUS not having start timestamp.

* Update default enums

Use an enum name suffix instead of prefix for the default values.

* Remove refinements

Co-authored-by: Tigran Najaryan <4194920+tigrannajaryan@users.noreply.github.com>
Co-authored-by: Bogdan Drutu <bogdandrutu@gmail.com>