Add onDropped callback for throttleFirst - addresses #7481

[Desislav-Petrov]

This PR adds an overload to the throttleFirst operator which takes an onDropped consumer.

This consumer is called when we're still within the gated time window when any new item isn't passed to downstream.

If the onDropped crashes, the exception is either delivered to the downstream (if the sequence is still live at that point) or to the global error handler (sequence is stopped).

Related: #7458

[akarnokd]

This will hang the sequence because the error is swallowed. Also there is no cleanup of the worker.

[Desislav-Petrov]

So we have to do downstream.onError(t) to propagate this, right? And .dispose() of the worker too?

[akarnokd]

Yes.

[Desislav-Petrov]

Should be addressed now, thx for the review

[codecov]

Codecov Report

Merging #7482 (4e79c22) into 3.x (159ae3b) will increase coverage by 0.02%.
The diff coverage is 77.41%.

❗ Current head 4e79c22 differs from pull request most recent head 7a2792a. Consider uploading reports for the commit 7a2792a to get more accurate results

@@             Coverage Diff              @@
##                3.x    #7482      +/-   ##
============================================
+ Coverage     99.52%   99.55%   +0.02%     
- Complexity     6788     6790       +2     
============================================
  Files           752      752              
  Lines         47586    47611      +25     
  Branches       6394     6396       +2     
============================================
+ Hits          47359    47398      +39     
+ Misses          107      104       -3     
+ Partials        120      109      -11     

Impacted Files	Coverage Δ
...operators/flowable/FlowableThrottleFirstTimed.java	94.02% <63.63%> (-5.98%)	⬇️
...ators/observable/ObservableThrottleFirstTimed.java	93.47% <70.00%> (-6.53%)	⬇️
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <100.00%> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <100.00%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	⬇️
...nternal/operators/observable/ObservableCreate.java	97.43% <0.00%> (-1.71%)	⬇️
...nternal/operators/parallel/ParallelSortedJoin.java	98.49% <0.00%> (-1.51%)	⬇️
...internal/operators/flowable/FlowableSwitchMap.java	98.58% <0.00%> (-1.42%)	⬇️
...a3/internal/operators/flowable/FlowableCreate.java	99.35% <0.00%> (+0.32%)	⬆️
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	99.58% <0.00%> (+0.62%)	⬆️
... and 6 more

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