[Feature Request]: Propagate drain mode up to reduceFn

[stankiewicz]

What would you like to happen?

Continuation of #36534

Information about the fact that aggregation or timer was induced by drain event is stored in windmill internal data strutures.
Propagate it to TimerData and up to ReduceFn

Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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[stankiewicz]

.take_issue