Add env variable for suppressing ASGI event spans from Starlette Instrumentor

What problem do you want to solve?

The OpenTelemetry middleware that Starlette uses internally has a setting to suppress ASGI event spans like receive and send: https://github.com/open-telemetry/opentelemetry-python-contrib/blob/86d26ce1b8d24299bcc17a074744bbe19d812732/instrumentation/opentelemetry-instrumentation-asgi/src/opentelemetry/instrumentation/asgi/__init__.py#L573.

However, the Starlette auto-instrumentation always emits those spans as it never sets that constructor value.:
https://github.com/open-telemetry/opentelemetry-python-contrib/blob/51da0a766e5d3cbc746189e10c9573163198cfcd/instrumentation/opentelemetry-instrumentation-starlette/src/opentelemetry/instrumentation/starlette/__init__.py#L246
https://github.com/open-telemetry/opentelemetry-python-contrib/blob/51da0a766e5d3cbc746189e10c9573163198cfcd/instrumentation/opentelemetry-instrumentation-starlette/src/opentelemetry/instrumentation/starlette/__init__.py#L322

While these spans are useful, they can sometimes create a lot of noise in the telemetry data.

Describe the solution you'd like

I'd like to propose adding an environment variable configuration: OTEL_PYTHON_STARLETTE_EXCLUDED_SPANS that lets users disable specific spans by passing them to the middleware. For example:

OTEL_PYTHON_STARLETTE_EXCLUDED_SPANS='receive,send'

This would allow users to filter out any ASGI event spans like receive and send if they're not needed.

Describe alternatives you've considered

N/A

Additional Context

N/A

Would you like to implement a fix?

None

Tip

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Start with the Starlette Instrumentor entry points in instrumentation/opentelemetry-instrumentation-starlette/src/opentelemetry/instrumentation/starlette/init.py at the referenced middleware construction lines, then compare the ASGI middleware setting in instrumentation/opentelemetry-instrumentation-asgi/src/opentelemetry/instrumentation/asgi/init.py. Add support for OTEL_PYTHON_STARLETTE_EXCLUDED_SPANS and verify that values such as receive,send suppress the corresponding ASGI event spans.