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