feat: new WriteOption config for capturing backpressure data

[fredjoonpark]

Closes #846

Proposed Changes

Introduces an (optional) new WriteOption config, captureBackpressureData, which captures dropped data points on backpressure events.

Additionally adds another config called concatMapPrefetch - previously, the client was calling concatMapMaybe with an undocumented prefetch value (source). This matters because prefetched batches are removed from the buffer and needs to be accounted for during testing. Made this configurable with the default set as what it's been using.

• added guards and missing tests for WriteOption boundary conditions (ie. buffer limit, exponential base)

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• mvn test completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.81%. Comparing base (dd37956) to head (e92640e).
⚠️ Report is 18 commits behind head on master.
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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #848      +/-   ##
============================================
+ Coverage     88.51%   88.81%   +0.29%     
- Complexity      787      791       +4     
============================================
  Files           174      174              
  Lines          7207     7248      +41     
  Branches        405      413       +8     
============================================
+ Hits           6379     6437      +58     
+ Misses          697      680      -17     
  Partials        131      131              

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[bednar]

@fredjoonpark thanks for fantastic PR, it's look completely OK. 👍

LGTM 🚀