Rename http.server.active_requests metric to http.server.current_requests

[trask]

Fixes #202

Note: this metric is not (currently) in our initial stability plan, but I'm wondering now if it should be, so that this breaking change can be covered by the OTEL_SEMCONV_STABILITY_OPT_IN flag.

EDIT: Nevermind, closing this PR, I think the naming convention here is still unclear. It would be good to have consistency across:

• jvm.thread.count
• jvm.class.count
• http.server.current_requests

which ties into #211 and #260 (comment).

Changes

Renames http.server.active_requests metric to http.server.current_requests.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml updated with changes to existing conventions.