Fix tracing decorator with late configuration

[oceyral]

Description

Related to #2621 and #2708

When using the tracing decorator documented as part of the issue and PR above, functions might not actually be instrumented when the decorator is used on a function before a tracing provider is configured.

Considering the following case :

from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider


tracer = trace.get_tracer(__name__)

def setup_tracing() -> None:
    provider = TracerProvider()
    processor = BatchSpanProcessor(ConsoleSpanExporter())
    provider.add_span_processor(processor)
    trace.set_tracer_provider(provider)

@tracer.start_as_current_span("toto")
def toto():
    print("toto")

def main():
    setup_tracing()
    toto()

if __name__ == '__main__':
    main()

Here, the toto function will not actually be instrumented, even after configuration of a tracing provider. This is particularly problematic with instrumenting submodules for example, because if the module is imported before a tracer provider is configured, most uses of the decorator will be no-ops, and no tracing will be done.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Added test case in opentelemetry-api/tests/trace/test_proxy.py

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: oceyral (734cc1a, 7960d2e)
• ✅ login: ocelotl / name: Diego Hurtado (e47d49d, 5ffd97a)

[ocelotl]

@oceyral please sign the CLA

[oceyral]

@ocelotl yes, I'm trying to navigate corporate structure to get validated as a contributor, sorry if it takes a bit of time

[oceyral]

@ocelotl got the CLA signed ! I also tried fixing mypy/lint tests, but I'm not sure about the other failing tests, I had a look but it doesn't look related to my PR.

[ocelotl]

@ocelotl got the CLA signed ! I also tried fixing mypy/lint tests, but I'm not sure about the other failing tests, I had a look but it doesn't look related to my PR.

Thanks, seems like this PR and others are failing because an unrelated infra issue which should be fixed by #2790.