Prevent RuntimeError on context.detach with the same token

This happens for me when using werkzeug's debug=True.
In this case, the token was already detached. But when I enter an
expression into the debug console, we attempt to detach the token once
more.

This causes a RuntimeError, as expected: https://peps.python.org/pep-0567/#contextvars-contextvar.

But if we set _ENVIRON_TOKEN to None after we detach, this won't happen.

Author: abo-abo

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

@@ -536,6 +536,7 @@ def _teardown_request(exc):
 
         if flask.request.environ.get(_ENVIRON_TOKEN, None):
             context.detach(flask.request.environ.get(_ENVIRON_TOKEN))
+            flask.request.environ[_ENVIRON_TOKEN] = None
 
     return _teardown_request