fix(flask): align http.server.active_requests metric with semconv helper (#4094)

* fix(flask): correct HTTP metrics handling with semconv opt-in

* fix(flask): avoid hardcoded legacy http.server.duration metric name

* changelog: note flask http server metrics consistency fix

* fix(flask): use semconv constant for legacy http.server.duration metric

* Update instrumentation/opentelemetry-instrumentation-flask/src/opentelemetry/instrumentation/flask/__init__.py

* Update CHANGELOG.md

* fix(flask): explicitly split active requests metric by semconv mode

* fix(flask): align http.server.active_requests metric with semconv helper

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Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>
Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>