DB and HTTP semantic convention stability migration for SQLAlchemy (#4110)

* Add _semconv set_db fns

* lint

* Changelog

* semconv opt-in for sqlalchemy instr

* use imports not hardcode

* Update comment on comment-in-attr

* Add tests

* Changelog

* Add migration status

* Sqlalchemy http,database opt-in

* meter schema_url

* Add sqlalchemy _get_db_name_from_cursor

* Add per-signal semconv opt-in schema url precedence; add sqlalchemy usage

* Update _get_db_name coverage

* More db_name_from_cursor cases

* split to lint

* _get_schema parses url from semconv

* _get_* from cursor or conn

* Add _semconv set_db_operation, used by sqlalchemy

* get_db_name safer, update docker-test

* Fix test

* Add schema_url asserts

* Update CHANGELOG.md

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>