Complete the metrics/api.md TODOs

[jmacd]

The TODOs in specification/metric/api.md should be finished for the 0.5 spec milestone.

api.md�316:TODO: move and update content on metric names from api-user.md
api.md�608:TODO: specify detail on the metric instrument constructors from api-user.md
api.md�612:TODO: incorporate more detail on label sets from api-user.md
api.md�616:TODO: Organize a section on synchronous instruments from:
api.md�648:TODO: Organize a section on asynchronous instrument from:
api.md�655:TODO: This content will be re-organized into the sections dedicated to

Most of the missing content was already settled in earlier specification versions, but the documents needed to be restructured. The former content lies here:

https://github.com/open-telemetry/opentelemetry-specification/blob/494bb6dbd38d6d801871f34e9d141184b9e86d07/specification/metrics/api-user.md

The only parts of the spec that still need to be "settled" by these changes are related to asynchronous instrument calling conventions, which are under review:

open-telemetry/opentelemetry-go#717
#549