Add environment variable for exclude_spans parameter in FastAPI/ASGI instrumentation

What problem do you want to solve?

When using auto-instrumentation (opentelemetry-instrument), there's no way to exclude the send and receive spans that the ASGI middleware generates. These spans are very noisy for typical REST APIs — a simple "hello world" endpoint generates 3 spans instead of 1, and the child http send/http receive spans rarely provide actionable information.
The exclude_spans parameter exists and works great when using programmatic instrumentation:

FastAPIInstrumentor.instrument_app(app, exclude_spans=["send", "receive"])

However, there's no environment variable equivalent, which means users of auto-instrumentation cannot use this feature without switching to manual instrumentation.

Describe the solution you'd like

Add an environment variable like OTEL_PYTHON_FASTAPI_EXCLUDE_SPANS (or OTEL_PYTHON_ASGI_EXCLUDE_SPANS for the base ASGI instrumentation) that accepts a comma-separated list of span types to exclude.
Example:

OTEL_PYTHON_FASTAPI_EXCLUDE_SPANS="send,receive"

This would be consistent with other env var patterns in the instrumentation like OTEL_PYTHON_FASTAPI_EXCLUDED_URLS.

Describe alternatives you've considered

Switching from auto-instrumentation to programmatic instrumentation just to set this one option — this defeats much of the convenience of auto-instrumentation.
Using a custom span processor to filter spans at export time — this works but is more complex and filters spans after they've already been created (wasted work).
Using a custom sampler to drop spans by name — same issue as above, plus it can cause problems (e.g., #2087 where http.status_code wasn't set on the server span when child spans were dropped).

Additional Context

Related issues where users have complained about the noisy send/receive spans:

#831
#635

The exclude_spans parameter was added to address this, but the lack of env var support limits its usefulness for auto-instrumentation users.

Would you like to implement a fix?

None

Tip

_{React with 👍 to help prioritize this issue. Please use comments to provide useful context, avoiding +1 or me too, to help us triage it. Learn more here.}

Start at FastAPIInstrumentor.instrument_app and trace how the existing OTEL_PYTHON_FASTAPI_EXCLUDED_URLS setting is configured for auto-instrumentation. Add equivalent comma-separated environment-variable support for exclude_spans, then verify that values such as "send,receive" prevent those ASGI spans while preserving programmatic behavior.