Do not configure instrumentation package config/env vars globally

Most instrumentation packages support some form of configuration via env vars and right now all packages read this configuration at import time into a global var. For example, https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/instrumentation/opentelemetry-instrumentation-flask/src/opentelemetry/instrumentation/flask/__init__.py#L76

This has a few obvious disadvantages such as:

• harder to test as it needs patching in multiple places.
• hard to run two tests that need different values in parallel.
• code is executed on import even if instrumentation is not used.
• force instrumentation to use global vars.

I suggest we fix this by moving such lines to instrumentor constructors so the value is read once per instrumentor application. This would simplify testing and localize all config logic related to an instrumentor inside its constructor.

We already did this for Falcon and as a result significantly simplified some test code: https://github.com/open-telemetry/opentelemetry-python-contrib/pull/650

We should do it for all other affected instrumentation packages.

Start with the Flask instrumentation init.py location linked in the issue, then compare the Falcon change in pull request #650. Identify other instrumentation packages that read configuration into globals at import time, and review their instrumentor constructors and tests. Done means affected packages read configuration per instrumentor application and their tests still pass without shared global state.