Add example for sleuth/brave integration

This is a usage question.

In the [Sleuth docs](https://cloud.spring.io/spring-cloud-sleuth/spring-cloud-sleuth.html#_grpc) for gRPC, they mention two variants. First one uses `io.github.lognet:grpc-spring-boot-starter` and `io.zipkin.brave:brave-instrumentation-grpc` and clients must use `SpringAwareManagedChannelBuilder`. In the second variant, nothing is said other than _"Grpc Spring Boot Starter automatically detects the presence of Spring Cloud Sleuth and brave’s instrumentation for gRPC and registers the necessary client and/or server tooling."_

There doesn't seem to be any relation between these projects, and are maintained by different people. `yidongnan/grpc-spring-boot-starter` doesn't seem to use `SpringAwareManagedChannelBuilder` anywhere. Should it? Also, does it need `io.zipkin.brave:brave-instrumentation-grpc` to work?

Perhaps the owners of the two projects should discuss merging them. It seems like a waste of time to develop two parallel projects for the same purpose.