Metrics API specification: incorporate old content on calling convent…

…ions, label sets. (open-telemetry#601)

* Add sections on MetricProvider and obtaining a Meter

* Add sections on Naming requirements and instrument constructors

* Calling conventions, remove TODOs

* Update TOC

* Reindent sets of labels

* Reindent end matter

* Whitespace

* Feedback

* feedback on Bogdan's review

* Spelling

* MetricProvider->MeterProvider

* feedback on jkwatson's review

* Whitespace lint

* respond to cijo's comments