AsgiSetter should override header values instead of appending

[phillipuniverse]

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.

opentelemetry-python-contrib/instrumentation/opentelemetry-instrumentation-asgi/src/opentelemetry/instrumentation/asgi/__init__.py

Lines 219 to 240 in fbac424

	class ASGISetter(Setter[dict]):
	def set(
	self, carrier: dict, key: str, value: str
	) -> None: # pylint: disable=no-self-use
	"""Sets response header values on an ASGI scope according to `the spec <https://asgi.readthedocs.io/en/latest/specs/www.html#response-start-send-event>`_.
	Args:
	carrier: ASGI scope object
	key: response header name to set
	value: response header value
	Returns:
	None
	"""
	headers = carrier.get("headers")
	if not headers:
	headers = []
	carrier["headers"] = headers
	headers.append([key.lower().encode(), value.encode()])
	asgi_setter = ASGISetter()

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 #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.