Add support for Reactor Netty Micrometer metrics

This commit enables the production of TCP and buffer allocator metrics
for Reactor Netty, client and server.

When applications use auto-configured server
(`NettyReactiveWebServerFactory`) and client (through
`WebClient.Builder`) instances, metrics will be enabled.

Note that HTTP metrics are not enabled here, since similar metrics are
already produced at the WebFlux level. Also, to avoid cardinality
explosion of metrics (through the URI tag), Reactor Netty offers
configurable infrastructure to deduplicate URI tags by turning expanded
URI instances into templated URIs. This is not targeted for Spring
usage.

Closes gh-19388

spring-boot-project/spring-boot-docs/src/docs/asciidoc/production-ready-features.adoc

@@ -1813,6 +1813,7 @@ Spring Boot registers the following core metrics when applicable:
 * Logback metrics: record the number of events logged to Logback at each level
 * Uptime metrics: report a gauge for uptime and a fixed gauge representing the application's absolute start time
 * Tomcat metrics (`server.tomcat.mbeanregistry.enabled` must be set to `true` for all Tomcat metrics to be registered)
+* Reactor Netty metrics (TCP and allocator metrics for client and server)
 * {spring-integration-docs}system-management.html#micrometer-integration[Spring Integration] metrics