Fix typing errors with set_extenion methods

[mribbons]

Description

eg:

Argument of type "(span: Span, tag: str) -> str" cannot be assigned to parameter "method" of type "SpanMethod | None" in function "set_extension"
  Type "(span: Span, tag: str) -> str" is incompatible with type "SpanMethod | None"
    "function" is incompatible with protocol "SpanMethod"
      "__call__" is not present
    "function" is incompatible with "None"PylancereportArgumentType

Rationale:
Callable only accepts a single parameter, optional parameters don't work
... isn't recognized as variadic arguments (... not valid here)
SomeMethod(Protocol) __call__ approach doesn't support mapping non variadic arguments to variadic arguments.

Therefore, to not break the currently documented interface, implement a union of Callable overloads.

Types of change

Typing fix

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[honnibal]

The current way does indeed look broken, but if we can't make it correct I'd rather just declare them as Callable? The union up to an arbitrary number isn't very satisfying.

[mribbons]

Hey @honnibal,

The union up to an arbitrary number isn't very satisfying.

I agree, I just didn't want to break the current API.

Turns out the docs in master use Callable anyway.

PR updated.

[RahulVadisetty91]

First method declaration is correct, second method is incorrect.