[Feature Request][Go SDK]: Window Aware CombineFns

[lostluck]

What would you like to happen?

There's been a quiet request to have windows also be providable to CombineFn methods.

Very similar to key aware CombineFns in issue #22211 in that it would require expanding permitted parameter orderings for combine methods, and ensuring they are properly populated at execution time.

Combiners are already implicitly window aware, so there's no change in performance as a result of this.

For both issues, the generic register.Combine methods would need to be expanded accordingly to handling the additional parameters to the methods, when present.

Example use case is being able to have the accumulator extract time bounds information from the window, such as for hooking into OpenTelemetry metrics downstream.

Issue Priority

Priority: 2

Issue Component

Component: sdk-go