AsgiSetter should override header values instead of appending

Describe your environment

Using FastAPI 0.79 and relying on the AsgiSetter. The AsgiSetter has a surprising result to always append values instead of overriding previous keys.

https://github.com/open-telemetry/opentelemetry-python-contrib/blob/fbac4245159e51877efa86581724995bd1415a18/instrumentation/opentelemetry-instrumentation-asgi/src/opentelemetry/instrumentation/asgi/__init__.py#L219-L240

Steps to reproduce

Call AsgiSetter().set() multiple times with the same header name.

What is the expected behavior?

The 2nd call with the same header appends to the previous header value.

What is the actual behavior?
The result will be multiple headers with the same name are added instead of a single header with multiple values.

Additional context

Originally added by me (😬 ) at https://github.com/open-telemetry/opentelemetry-python-contrib/pull/817. It's also not 100% clear what the right behavior should be. Need to investigate the ASGI spec and what happens with multiple tuples with the same header name.

Start with instrumentation/opentelemetry-instrumentation-asgi/src/opentelemetry/instrumentation/asgi/init.py around lines 219-240 and review the ASGI specification for repeated header names. Reproduce multiple AsgiSetter().set() calls with the same header name, then define the behavior that matches the specification and verify the implementation follows it.