Integrate the Micrometer Context Propagation library

[rstoyanchev]

The Micrometer context-propagation library propagates context across different types of context such as ThreadLocal, Reactor Context, and others. It does not replace those but rather helps to propagate values from one type of context to another, or simply to restore ThreadLocal values on a different thread.

One place where we can integrate the library is in Spring MVC. When a controller method returns Flux or Mono, we can use the Context Propagation library to populate the Reactor Context with values from any ThreadLocal's for which there is a registered ThreadLocalAccessor.

[rstoyanchev]

The change is ready but needs to be postponed due to a classpath conflict between context-snapshot version 1.0 M5 vs 1.0 M3 brought in by Reactor Netty 2.0 M1 which in turn is waiting for a new Netty 5.0 Alpha5.