Update WebClient metrics auto-configuration to the new Observation instrumentation

[bclozel]

Since spring-projects/spring-framework#28341, Spring Framework now instruments directly WebClient, removing the need for a custom ExchangeFunction.

With this issue, we should update the auto-configuration to only rely on the new ObservationRegistry and configure the instrumentation available on WebClient. We need to provide a smooth upgrade experience: deprecate the tags providers and adapt them to the new infrastructure until it's completely removed.

[bclozel]

As part of this change, we've deprecated DefaultWebClientExchangeTagsProvider, WebClientExchangeTagsProvider and WebClientExchangeTags.

If an application needs to change the KeyValues (formerly Tags) for this observation, you can contribute a bean of type GlobalObservationConvention for the observation context we're considering.
Instead of inheriting from DefaultWebClientExchangeTagsProvider, we can here extend the DefaultClientObservationConvention and override the methods we want.

Here, let's add a "httpMethod" keyvalue by extracting the value from the request:

public class CustomClientObservationConvention extends DefaultClientObservationConvention
		implements GlobalObservationConvention<ClientObservationContext> {

	@Override
	public KeyValues getLowCardinalityKeyValues(ClientObservationContext context) {
		KeyValues keyValues = super.getLowCardinalityKeyValues(context);
		ClientRequest request = context.getCarrier();
		return keyValues.and("httpMethod", request.method().name());
	}

}