Flask instrumentor logs an error message when used with app.test_request_context

[jpmelos]

Describe your environment

You should be able to see the same result regardless of what the environment is.

Steps to reproduce

Python 3.6.9 (default, Apr 15 2020, 09:31:57) 
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IPython 7.16.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: from opentelemetry.instrumentation.flask import FlaskInstrumentor
   ...: FlaskInstrumentor().instrument()
   ...: 
   ...: import flask
   ...: 
   ...: app = flask.Flask(__name__)
   ...: 
   ...: with app.test_request_context():
   ...:     print('Hey!')
   ...: 
Hey!
Flask environ's OpenTelemetry activation missingat _teardown_flask_request(None)

What is the expected behavior?

I did not expect to see the error message in the last line of the example.

What is the actual behavior?

I see the line.

Additional context

According to Flask’s documentation, app.test_request_context does not run before_request and after_request handlers, but it does run teardown_request handlers, which means the counter-handler of before_request is after_request, not teardown_request. Source: https://flask.palletsprojects.com/en/1.1.x/testing/#other-testing-tricks (5th paragraph after the anchor).

The Flask instrumentor doesn't take that into consideration, and it registers a handler with before_request, and expects it to have run when the teardown_request handler runs, which is not guaranteed: handlers are created[1][2] and handler's are removed. Because the before_request handler won't run for app.test_request_context, it won't have flask.request.environ['_ENVIRON_ACTIVATION_KEY'], which is required here to close the span that should have been opened in the before_request handler that never ran.