Rename http.*.duration metrics to http.*.request.duration

[lmolkova]

At some point, we might want to report the connection duration, body stream reading duration, or something else in addition to the request duration.

If this happens, we'd regret having ambiguous http.client|server.duration.

Proposal:

• rename http.client.duration to http.client.request.duration
• rename http.server.duration to http.server.request.duration

Context: .NET runtime is being instrumented with HTTP metrics and will report both: HTTP connection duration and standard request durations (lmolkova#1)