suppress_instrumentation utils are not used consistently

[alexmojaki]

Searching for suppress_instrumentation (https://github.com/search?q=repo%3Aopen-telemetry%2Fopentelemetry-python-contrib%20suppress_instrumentation&type=code) reveals places using the old plain string "suppress_instrumentation" context key, e.g:

opentelemetry-python-contrib/instrumentation/opentelemetry-instrumentation-httpx/src/opentelemetry/instrumentation/httpx/__init__.py

Line 350 in f4f3042

if context.get_value("suppress_instrumentation"):

Since this is not how instrumentation is actually suppressed any more, this can lead to bugs.

There are also places checking/setting the key 'manually' instead of via the utility functions, which also seems error-prone.

Additionally, if it's allowed for a user to have different versions of opentelemetry-instrumentation and opentelemetry-instrumentation-httpx (or another specific library instrumentation package) installed, then the util functions suppress_instrumentation and is_instrumentation_enabled should also check the plain string key "suppress_instrumentation" for backward compatibility.

[xrmx]

Nice catch, a PR would be great.