Add guidance for start and stop points for Http Server span/duration metric

[tylerbenson]

Area(s)

area:http

What's missing?

In the client span spec, there's some guidance on when the span should start/stop, and the expectation that the duration metric roughly follows.
Similar guidance is missing on the Http Server side of the spec.

Describe the solution you'd like

I'd like to see clarification on when the span/metric should be started and stopped. Starting is a bit more obvious and intuitive (as soon as the request headers are parsed that the http server framework allows), but the end is less clear. Most comprehensive would be when the last response body byte is sent, but I have seen arguments for after the headers are committed and before the response body is sent. (Perhaps the specification should instead add that point as a span event.)

[trask]

adding slack comment from @jsuereth:

I think the "generic" http server span should represent the entire time taken to process the request (from parsing to last byte)