Document instrumentation for RestTemplate and/or WebClient

[oppegard]

Affects: 6.0.x

---

The following callout was written in the Spring Sleuth documentation for HTTP clients:

You have to register RestTemplate as a bean so that the interceptors get injected. If you create a RestTemplate instance with a new keyword, the instrumentation does NOT work.

I can't figure out if the same guidance applies in 6.0.x for Micrometer Tracing at either:

• https://docs.spring.io/spring-framework/docs/6.0.4/reference/html/web.html#webmvc-client
• https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#actuator.micrometer-tracing

[bclozel]

I'll create a follow-up issue in Spring Boot to do the same.