Add details for Aggregator in Metrics Spec.

[victlu]

Add more details for an Aggregator in the Metrics Spec.

This PR is a rework of #1804 after discussions and feedback from metric SIG meetings.

[pirgeo]

Flagged a few typos, but overall I think it is very clear and concise 👍

[jsuereth]

Want to confirm a few points that I think are implicit in this PR that perhaps deserve some in person discussion:

1. Default aggregation for synchronous instruments is DELTA right now, not cumulative.
2. This PR does not define any kind of "back-hook" where exports can ask for a specific type of temporal aggregation (delta vs. cumulative). The configuration of the View (or default) determines the temporarlity.
3. The public names exposed in SDK to users are "GAUGE", "SUM" and "HISTOGRAM", vs. the more specific name used in the section.

[jmacd]

Thank you @victlu. I know this has been a lot of work!

[reyang]

LGTM.

[jmacd]

@cijothomas are you happy with the edits that @victlu has made? This looks ready to merge.

[cijothomas]

nit: since we don't specify any "how" here, "ways and means" sounds too much.

[cijothomas]

left small editorial comments.