Explain the Metric API instruments (open-telemetry#98)

* Nine instruments

* WIP: More explanation, starting on details section

* Introduction

* Typo fix

* Table fmt

* Rewrite without async delta instruments

* More examples

* Draft is ready

* More names considered

* More examples from review feedback

* From comments

* Typos

* Address question about MMSC

* About ValueObserver temporal quality

* More on temporal quality terminology

* 88 links to otep 88

* Swap temporal quality and default aggregation columns

* Move link ref

* Add 'Input' to the temporal quality header

* Add detail

* Rename into ./metrics

* Lint

* Lint