Add maxMessages to PubSubReactiveFactory.poll

[mturbe]

Fixes #2072

[codecov]

Codecov Report

Merging #2441 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #2441      +/-   ##
============================================
+ Coverage     74.10%   74.14%   +0.03%     
- Complexity     2121     2123       +2     
============================================
  Files           264      264              
  Lines          7654     7657       +3     
  Branches        792      792              
============================================
+ Hits           5672     5677       +5     
+ Misses         1619     1617       -2     
  Partials        363      363              

Flag	Coverage Δ	Complexity Δ
#unittests	74.14% <100.00%> (+0.03%)	2123.00 <4.00> (+2.00)

Impacted Files	Coverage Δ	Complexity Δ
...oud/gcp/pubsub/reactive/PubSubReactiveFactory.java	88.09% <100.00%> (+0.91%)	16.00 <4.00> (+2.00)
...tastore/GcpDatastoreEmulatorAutoConfiguration.java	72.41% <0.00%> (+6.89%)	6.00% <0.00%> (ø%)

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[elefeint]

Thank you for the help! Your PR is very well done.
I'd like @meltsufin 's take tomorrow on how to acquire max messages; depending on the outcome you may either be able to merge this version (with a small validation change) or update the logic to accept maxMessages as a constructor argument.

[mturbe]

@elefeint, @dzou, @meltsufin: thank you for the feedback. I have reverted my previous commit and added a new overloaded constructor for PubSubReactiveFactory that has maxMessages as a parameter.

[meltsufin]

Thanks @mturbe!

[elefeint]

Thank you!

[elefeint]

As follow-up, I've filed #2451 to autoconfigure the new property.