RUM-6171 use FileObserver to limit calls to filesystem

[0xnm]

What does this PR do?

The same as #2718, just branch moved directly to the repo.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.31%. Comparing base (7606b1a) to head (441a603).

Files with missing lines	Patch %	Lines
...al/persistence/file/batch/BatchFileOrchestrator.kt	95.92%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2720      +/-   ##
===========================================
+ Coverage    71.28%   71.31%   +0.03%     
===========================================
  Files          880      880              
  Lines        32360    32372      +12     
  Branches      5457     5461       +4     
===========================================
+ Hits         23066    23083      +17     
- Misses        7736     7745       +9     
+ Partials      1558     1544      -14     

Files with missing lines	Coverage Δ
...al/persistence/file/batch/BatchFileOrchestrator.kt	94.61% <95.92%> (+1.71%)	⬆️

... and 36 files with indirect coverage changes

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[datadog-datadog-prod-us1]

🎯 Code Coverage
• Patch Coverage: 94.12%
• Overall Coverage: 66.37% (+0.00%)

View detailed report

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[0xnm]

Looks good! It is worth to do some benchmarks to compare the old and new ways of handling batch files in different scenarios. For example, when there is no batch files, 100 batch files, 10000 batch files.

[0xnm]

In theory CAS operation isFileObserverStarted.compareAndSet(false, true) already guards the logical branch from parallel execution, so @Synchronized is not needed. Do I miss something?

[0xnm]

This will be called from getRootDir <- ... <- SdkFeature.prepareStorage, meaning it will be called on the caller thread. In most of the cases it will be main thread during app startup, so we need to make sure that these changes don't have significant impact perf-wise at the SDK initialization.