`SanitizeValue.sanitize_header_values()` type signature is incorrect

[aabmass]

          > **Subtlety:** [the docs](https://opentelemetry.io/docs/specs/semconv/http/http-spans/#:~:text=is%20not%20recommended.-,The%20attribute%20value%20MUST%20consist%20of%20either%20multiple%20header%20values%20as%20an%20array%20of%20strings%20or%20a%20single%2Ditem%20array%20containing%20a%20possibly%20comma%2Dconcatenated%20string%2C%20depending%20on%20the%20way%20the%20HTTP%20library%20provides%20access%20to%20headers.,-%5B2%5D%3A%20The) allow you to use either a list of header values where the keys are repeated or a single item list with the values comma separated.

In the case of WSGI request headers which come pre-joined, I think it makes sense to leave them comma separated. For WSGI response headers and ASGI overall, where we have access to repeated headers, it might make sense to use the list for the reasons you mentioned. I think that is the sprit of the spec when it says "depending on the way the HTTP library provides access to headers".

this would means changing the signature of numerous public methods
and, changing the docstring which perversely (presumably according to some old spec) specificly say you'll get a single item list with values comma seprated

@ocelotl @xrmx any thoughts on this?

I was actually confused reading the code, because It looks like this signature for SanitizeValue.sanitize_header_values() is already wrong (it's returning a dict[str, list[str]] 😓 Were there other signatures?

Originally posted by @aabmass in #2266 (review)

[samuelcolvin]

Fix proposed in #2361.

[xrmx]

Subtlety quote is:
The attribute value MUST consist of either multiple header values as an array of strings or a single-item array containing a possibly comma-concatenated string, depending on the way the HTTP library provides access to headers.