Allow ServerHttpObservationFilter to be extended

[szpak]

Spring (Boot) with micrometer(-tracing) nicely propagate traceparent from an HTTP request header into traceId/spanId withing the app (MDC, etc). However, no traceparent header is included in the Spring (Boot) controller response (corresponding to that request).

While that feature could be implemented with the Filter, after a discussion with @jonatan-ivanov on Micrometer's Slack, I propose to have this available as a feature in (Reactive) Spring Web.

For backward compatibility, it could be initially available as opt-in, however, for a request with traceparent, it is natural to have it included also in a corresponding response.

[jonatan-ivanov]

I think this feature can be useful for clients who want to report issues against application they call, e.g.: an error or a slow response. This information for example can be added to error messages on UIs so that it can be much simpler to troubleshoot issues when users report it.

Also, it's important to call out that headers in the response is not necessary for context propagation, I think this feature is more similar to log correlation (traceId/spanId in log entries). Because of this, it is not a must to send back the traceparent header, also users can use other propagation formats, not necessarily W3C (traceparent) but sending back the tracing information in some format (or letting the user define that format) would be really nice.

[jonatan-ivanov]

A related feature to this could be adding the tracing information to ProblemDetails/error response (as the article suggests above).

[arung0wda]

Is this feature available? How do I use it?

[jonatan-ivanov]

Right now it is not, you need to implement it, there is an example in the PR description.
Here's another one, this is what I'm doing to add the trace info to the ProblemDetails (only for error response): CommonExceptionHandler.