… end (linkerd#589)

This PR adds the `request_duration_ms` metric to the Prometheus metrics exported by the proxy. It also modifies the `request_total` metric so that it is incremented when a request stream finishes, rather than when it opens, for consistency with how the `response_total` metric is generated.

Making this change required modifying `telemetry::sensors::http` to generate a `StreamRequestEnd` event similar to the `StreamResponseEnd` event. This is done similarly to how sensors are added to response bodies, by generalizing the `ResponseBody` type into a `MeasuredBody` type that can wrap a request or response body. Since this changed the type of request bodies, it necessitated changing request types pretty much everywhere else in the proxy codebase in order to fix the resulting type errors, which is why the diff for this PR is so large.

Closes linkerd#570