Encourage the use of third-party tracing and observability solutions in addition to / instead of implementing your own HttpTraceRepositroy

[wilkinsona]

Following on from #15039, we should update the reference documentation to suggest the use of third-party tracing/observability solutions (such a Zipkin or Spring Cloud Sleuth) as an alternative to Boot's HTTP tracing and implementing your own repository.

We could add it here where we currently say "for production environments, consider creating your own alternative HttpTraceRepository implementation".