docs(config): Document ObjectMapper config

[MikeGoldsmith]

Summary

Documents ObjectMapper configuration for external consumers via copy-paste approach instead of exposing internal API. Per maintainer feedback, avoids coupling by providing documentation rather than API access.

Addresses #7843

Background

Spring starter needs same ObjectMapper config to parse OpenTelemetry YAML. Three approaches considered:

1. Copy-paste (current in instrumentation)
2. Reflection
3. Internal API (original PR approach)

Changes

• Added "For Implementers" section to DeclarativeConfiguration javadoc with complete ObjectMapper setup example
• Added field-level javadoc on MAPPER explaining configuration
• Documented why each setting exists (empty objects for nulls, boxed primitives stay null)

Testing

• No API changes, only documentation additions

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.21%. Comparing base (f42ac7d) to head (c872957).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8075      +/-   ##
============================================
+ Coverage     90.20%   90.21%   +0.01%     
- Complexity     7593     7606      +13     
============================================
  Files           841      841              
  Lines         22913    22923      +10     
  Branches       2289     2291       +2     
============================================
+ Hits          20668    20680      +12     
+ Misses         1529     1526       -3     
- Partials        716      717       +1     

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[jack-berg]

@zeitlinger / @trask is this still needed from the instrumentation side? The last comment in the issue indicates a preference for copy / paste rather than letting consumers leverage the same object mapper:

I personally prefer a bit of copy-paste over relying on internal APIs

My perspective: in addition to not relying on internal APIs, I don't like being in the business of having an API (public or internal) to access a jackson ObjectMapper. That's not our business.

[MikeGoldsmith]

@jack-berg thanks for feedback. I misunderstood the original ask, so sticking with simple docs approach is good.

I've updated the PR to just document how to configure the ObjectMapper. Let me know what you think 😄