Add local and tag correlation fields

[adispezo]

Currently with the following config example it's not possible to use BaggageField.getByName, BaggageField.updateValue for example.

management:
    tracing:
        baggage:
            correlation:
                fields: 
                    - some_local_field
                    - some_remote_field
           remote-fields:
               - some_remote_field

Only the "some_remote_field" can be retrieved and updated.

With the code proposed also the "local" correlation fields will be accessible.

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[mhalbritter]

Hello,

thanks for the PR! I've added some comments.

[mhalbritter]

Hey @adispezo , thanks for the changes! I'll get some feedback from the observability team on the local field change in Brave and then come back to you.

[mhalbritter]

Looks good, thank you!

Note to the boot team: when merging, we need something like this for OpenTelemetry, too.

[marcingrzejszczak]

There's no way to do it easily with OTel. What currently is being done is that all baggage gets extracted / injected (e.g.https://github.com/open-telemetry/opentelemetry-java/blob/main/api/all/src/main/java/io/opentelemetry/api/baggage/propagation/W3CBaggagePropagator.java#L50). We would need to wrap their baggage propagation mechanism into our one that would modify the current baggage not to contain local keys and then delegate to their code.

[mhalbritter]

Hm :( In this case, we won't support local fields when using OpenTelemetry. We could issue a log WARN statement if local fields are configured and OTel is used.

[mhalbritter]

I have some polished changes and tagged fields for Otel here: https://github.com/mhalbritter/spring-boot/tree/pr/37435

[mhalbritter]

Before merging this, #38724 needs to be considered.

[mhalbritter]

Merged, thank you. To accommodate the changes from #38724, i had to refactor a bit.