Cache request values when hosting metrics are enabled

[JamesNK]

Fixes #55280

Hosting metrics use data on HttpContext when writing tag values. For example, the HTTP method of a request is metadata when writing metrics.

However, the context is mutable, which means that someone can modify the values used by hosting metrics during the request pipeline. This has the worst impact on the active requests counter, which depends on tags being the same for the start and the end of the request. If tags don't match then you end up with two dimensions, one going up and the other going down:

http.server.active_requests{ http_request_method="GET", url_scheme="http"} = 100
http.server.active_requests{ http_request_method="GET", url_scheme="https"} = -100

Fix is to stash the initial values on the context (this PR puts them on the metrics tags feature which is on the context) when the request starts and then use the stashed values at the end of the request.

There is slightly more memory used per request when metrics are enabled (3 additional fields), and no impact when metrics are disabled.

[mgravell]

LGTM