I have sleuth defined in my spring boot application. It prints messages as in below example.
2021-02-06 21:11:37.661 DEBUG [,250addg195246a3,250addg195246a3] 11260 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet : Completed 200 OK
Since the question does not contain too many details, I'm assuming a bunch of things here:
How can I add more info to be printed from DispatcherServlet?
I guess you are referring to the tracing-info part ([,250addg195246a3,250addg195246a3]). Check this answer to understand how tracing data is injected there, by overriding the logging.pattern.level, you will be able to inject custom things there.
How can I create JFR (Java flight recorder) event with tracing info?
You can inject a Tracer and get the current span from it, you can inject the data held by the span.
Where exactly in DispatcherServlet it creates this message - I'd like to update it via instrumentation
This depends on what version are you using, you should be able find it by reading/debugging it.