Warn on Using Span Inside Generator Functions

[dhruv-ahuja]

Addresses the concerns mentioned in #1424, ensuring the span calls raise runtime warnings with apt documentation link, when used inside generator functions.

Test script output:

uvp spanner.py                                      
/Users/dhruv/coding/python/projects/oss/logfire/logfire/_internal/main.py:204: RuntimeWarning: Span is inside a generator function. See https://logfire.pydantic.dev/docs/reference/advanced/generators/#move-the-span-outside-the-generator.
  warnings.warn(
21:10:07.772 gen3
             │ spanner.py:28
21:10:07.773   usage within gen3 (manual span)
             │ spanner.py:29 info
/Users/dhruv/coding/python/projects/oss/logfire/logfire/_internal/main.py:204: RuntimeWarning: Span is inside a generator function. See https://logfire.pydantic.dev/docs/reference/advanced/generators/#move-the-span-outside-the-generator.
  warnings.warn(
21:10:07.774 async_gen
             │ spanner.py:41
21:10:07.774   usage within async_gen (manual span)
             │ spanner.py:42 info
Failed to detach context
Traceback (most recent call last):
  File "/Users/dhruv/coding/python/projects/oss/logfire/spanner.py", line 43, in async_gen
    yield
GeneratorExit

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/dhruv/coding/python/projects/oss/logfire/.venv/lib/python3.13/site-packages/opentelemetry/context/__init__.py", line 155, in detach
    _RUNTIME_CONTEXT.detach(token)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^
  File "/Users/dhruv/coding/python/projects/oss/logfire/.venv/lib/python3.13/site-packages/opentelemetry/context/contextvars_context.py", line 53, in detach
    self._current_context.reset(token)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^
ValueError: <Token var=<ContextVar name='current_context' default={} at 0x104a39170> at 0x10746adc0> was created in a different Context
Exception ignored in: <async_generator object async_gen at 0x1073729b0>
Traceback (most recent call last):
  File "/Users/dhruv/coding/python/projects/oss/logfire/spanner.py", line 49, in <module>
    asyncio.run(anext(async_gen()))
StopAsyncIteration: