Fix psycopg2 instrument_connection AttributeError

[tammy-baylis-swi]

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Description

Removes the attempt to set attribute connection._is_instrumented_by_opentelemetry because psycopg2.extensions.connection does not allow setting of extra attributes and we get AttributeError. Similar AttributeError also happened with setattr for connection._otel_orig_cursor_factory so also removed.

Adds todos to re-add checks for connection-already-being-instrumented later, as discussed:

# TODO Add check for attempt to instrument a connection when already instrumented
#      https://github.com/open-telemetry/opentelemetry-python-contrib/issues/3138

Fixes https://github.com/open-telemetry/opentelemetry-python-contrib/issues/2522

Type of change

Please delete options that are not relevant.

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

• [X] tox -e py312-test-instrumentation-psycopg2
• [X] Instrumented db client app that calls Psycopg2Instrumentor.instrument_connection
• [X] Instrumented db client app that calls Psycopg2Instrumentor.instrument

Does This PR Require a Core Repo Change?

• [ ] Yes. - Link to PR:
• [X] No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• [X] Followed the style guidelines of this project
• [x] Changelogs have been updated
• [X] Unit tests have been added
• [X] Documentation has been updated