docs: document the HTTP semconv rename and how to migrate (#14756)

* docs: document the HTTP semconv rename and how to migrate

The stable-semconv fix renamed the HTTP server metrics, so any dashboard or
alert built on the old names stops matching. The OpenTelemetry page said what
Langflow emits today but never said what it used to emit or how to get from one
to the other.

Includes the unit change on the duration metric, milliseconds to seconds. That
one is worth calling out on its own: a threshold carried over unchanged is out
by a factor of 1000, and it does not look broken, it just stops firing.

Also documents OTEL_SEMCONV_STABILITY_OPT_IN=http/dup for running both
generations during a migration, which works because the opt-in is a setdefault
and an operator-provided value wins.

Salvaged from #14335, which is otherwise superseded by this page. That draft
predates the rename landing and was the only place this was written down.

* docs: correct the http.target migration mapping

http.target does not become http.route. Per OpenTelemetry's HTTP migration
guide it splits into url.path and url.query, and http.route is listed as "no
change": a separate attribute that existed before and after.

The wrong mapping would have sent someone repointing an http.target dashboard
at templated routes instead of concrete paths, which is different data at a
different cardinality, so the note now says that explicitly.

Inherited from #14335 and carried over without checking. Found in review.