Codecov Report

Merging #4608 (946423b) into main (21abfa2) will decrease coverage by 0.00%.
The diff coverage is 94.44%.

@@             Coverage Diff              @@
##               main    #4608      +/-   ##
============================================
- Coverage     90.06%   90.06%   -0.01%     
- Complexity     5063     5069       +6     
============================================
  Files           583      583              
  Lines         15629    15646      +17     
  Branches       1500     1501       +1     
============================================
+ Hits          14077    14091      +14     
- Misses         1098     1100       +2     
- Partials        454      455       +1     

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Just so I understand, the difference between this and addPropertiesSupplier(Supplier<Map<String, String>> is that this allows you to conditionally change properties based on the presence of some config in ConfigProperties?

Yeah, that's exactly the point.
I believe that will be super useful for vendor distributions, where you might want to introduce a "synthetic" configuration flag that turns on/off a bunch of options at once (e.g. enable/disable all metrics), or append an auth header to OTLP headers without overwriting the user settings, etc.

Just a final gut-check: Are we ok with owning this API forever? We're adding it to a stable module, so the name, etc is going to be un-changable.

I think its reasonable. It overlaps with addPropertiesSupplier(Supplier<Map<String, String>>) and in retrospect I wish we could only have this new customizer instead of both. But the ability to layer configuration instead of overwriting it is definitely valuable: Imagine you want to have customizer that adds an API key header for authentication. With the addPropertiesSupplier route this would override or be overridden by user configuration for additional headers. In contrast with the customizer, you can notice that the user has added custom headers, and append an addition API key header.