test: Add test to confirm programmatic config wins over env in BatchConfig

[cijothomas]

Part of #1225
For BatchLogProcessor, we were already preferring code over env_variables, so there is no change required. I added tests to confirm the behavior, and included this information in the docs too.

This is just the start, we need to audit entire ENV variables in the repo, and make changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.7%. Comparing base (51dda2f) to head (5773fc9).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2781   +/-   ##
=====================================
  Coverage   79.7%   79.7%           
=====================================
  Files        123     123           
  Lines      23117   23136   +19     
=====================================
+ Hits       18428   18447   +19     
  Misses      4689    4689           

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[lalitb]

Thanks for adding the test. Similar to be added for Span Processor too. LGTM once this is fixed - https://github.com/open-telemetry/opentelemetry-rust/pull/2781/files#r1989799553