This commit adds support for Actuator Metrics for WebClient.
This support mirrors the current behavior for `RestTemplate`, reusing
the same metric name `"http.client.requests"` and tags.
`WebClient` is instrumented by a `MetricsWebClientFilterFunction` which
is applied by a `WebClientCustomizer`. This instrumentation happens
automatically only if you create an instance of `WebClient` using an
auto-configured `WebClient.Builder` bean.
This infrastructure is reusing de facto the `MeterFilter` that has been
added for `RestTemplate` in order to limit the "uri" tag cardinality.
Closes gh-12228
1ef0098ab5