completed SwitchOnFirst impl

[OlegDokuka]

porting of the SwitchOnFirst operator's implementation with no fusion for now. This is a complete, tested, merge-ready implementation. Fuseable part will be submitted separately since it is a little bit tricky

[codecov-io]

Codecov Report

Merging #1423 into master will decrease coverage by 0.09%.
The diff coverage is 73.41%.

@@             Coverage Diff             @@
##             master    #1423     +/-   ##
===========================================
- Coverage     84.27%   84.18%   -0.1%     
- Complexity     3895     3910     +15     
===========================================
  Files           358      359      +1     
  Lines         29435    29760    +325     
  Branches       5464     5527     +63     
===========================================
+ Hits          24806    25052    +246     
- Misses         3030     3078     +48     
- Partials       1599     1630     +31

Impacted Files	Coverage Δ	Complexity Δ
...ore/src/main/java/reactor/core/publisher/Flux.java	98.73% <100%> (ø)	511 <1> (+1)	⬆️
...rc/main/java/reactor/core/publisher/Operators.java	79.79% <63.15%> (-0.33%)	120 <2> (+3)
...java/reactor/core/publisher/FluxSwitchOnFirst.java	73.98% <73.98%> (ø)	3 <3> (?)
.../java/reactor/core/publisher/BlockingIterable.java	77.08% <0%> (-2.09%)	7% <0%> (ø)
...ava/reactor/core/publisher/WorkQueueProcessor.java	69.42% <0%> (-1.66%)	19% <0%> (ø)
.../java/reactor/util/concurrent/MpscLinkedQueue.java	88.15% <0%> (-1.32%)	17% <0%> (-1%)
...r-core/src/main/java/reactor/core/Disposables.java	88.95% <0%> (-1.23%)	21% <0%> (ø)
...java/reactor/core/publisher/FluxBufferTimeout.java	77.85% <0%> (-0.68%)	3% <0%> (ø)
...c/main/java/reactor/core/publisher/FluxExpand.java	94.61% <0%> (-0.45%)	3% <0%> (ø)
...ain/java/reactor/core/publisher/FluxPublishOn.java	86.59% <0%> (-0.21%)	6% <0%> (ø)
... and 19 more

---

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[simonbasle]

@OlegDokuka we'll hopefully have time to review in depth for 3.2.4 release, won't be able to in time for thursday's 3.2.3, but we haven't forgotten about this PR 😉

[OlegDokuka]

@OlegDokuka we'll hopefully have time to review in depth for 3.2.4 release, won't be able to in time for thursday's 3.2.3, but we haven't forgotten about this PR 😉

Hope this time it will not be like with AsyncFileFlux ;)

[bsideup]

Since it took a bit of time to understand this operator, here is a link to the original PR where this operator was added to rsocket-java:
rsocket/rsocket-java#435 (comment)

I find the description quite informative:

Added SwitchTransform to internal, useful when you need to look at the first message on a channel before deciding how to handle it.

[simonbasle]

please add more commit as you fix the review elements @OlegDokuka, we'll rebase/squash in the final step (or I'll squash when merging)

[simonbasle]

Thanks for the changes! I found another small test issue (assumption instead of assertion), plus it is still missing some documentation and a test for toConditionalSubscriber, otherwise LGTM

[OlegDokuka]

@simonbasle WIP. I'm traveling right now, so trying to push commits in transition 😄

[simonbasle]

please don't include a marble yet, there is an upcoming PR making contribution of marbles much easier and much more likely to produce similar-looking marbles.

Can you please remove both image files from this PR and add the operator to the list of missing SVG (see comment on the javadoc URL)?

Other issues listed in comments.