move declarative config ObjectMapper instance to internal class

[zeitlinger]

Is your feature request related to a problem? Please describe.

Spring starter with declarative config needs to use the same settings for object mapper as https://github.com/open-telemetry/opentelemetry-java/blob/e6584623ccd2c626d66b357fd7c2c837a9c862bb/sdk-extensions/incubator/src/main/java/io/opentelemetry/sdk/extension/incubator/fileconfig/DeclarativeConfiguration.java#L66

See https://github.com/open-telemetry/opentelemetry-java-instrumentation/pull/14062#discussion_r2527602568

Describe the solution you'd like

Move the instance to an internal class where it can be read without reflection.

Describe alternatives you've considered

Read the instance using reflection.

[trask]

any other options? it would be nice not to introduce new reliance on internals where possible (#6970)

[zeitlinger]

any other options? it would be nice not to introduce new reliance on internals where possible (#6970)

This is what I can think of - there really is a strong reliance on the jackson settings after all:

• copy-paste (what we have now)
• reflection
• internal class

[trask]

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