Can't use side inputs with Combine.PerKeyWithHotKeyFanout

[damccorm]

PerKeyWithHotKeyFanout doesn't allow a user to specify a side input and access it in the CombineFn. When the function withHotKeyFanout is called, any sideinputs stored in the Combine.PerKey are dropped. (https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/Combine.java#L1571): 

    public PerKeyWithHotKeyFanout<K, InputT, OutputT> withHotKeyFanout(
        SerializableFunction<?
super K, Integer> hotKeyFanout) {
      return new PerKeyWithHotKeyFanout<>(fn, fnDisplayData, hotKeyFanout);

   }

We would like to be able to use side inputs and hot key fanout.

Imported from Jira BEAM-11005. Original Jira may contain additional context.
Reported by: miagilepner.

[blazingbhavneek]

Hey there! 👋 I'm new to this repository and eager to contribute! 🌟 Could you kindly suggest some entry point or files to look into?

[damccorm]

Hey, saw you added this comment several places. I'd recommend focusing on a single issue at first (I answered the underlying question here - #20298 (comment))

[marc7806]

.take-issue