Provide a way to listen to specific services instead of the whole catalog

spring-cloud-consul provides a Consul Catalog Watch that publishes heartbeat events on catalog changes. In an environment with many services the catalog can change rapidly (multiple times per second) causing heartbeat events to trigger for services which the application is not interested in.

For example, this mechanism is used in Spring Cloud Config Client if discovery is enabled link

In practice the config client is only interested in updates to the spring-cloud-config-server but this triggers each time the catalog updates.

The catalog services watch can already be disabled. It would be very useful if an alternative heartbeat event producer can be implemented which takes a list of relevant services and only publishes heartbeat events when those specific services are updated.

Currently any micro service in our organization will start generating many requests to consul just by adding spring-cloud-starter-consul-discovery. While the watch-delay is configurable it's less than ideal if your application is only interested in a subset of services. If the watch-delay is too high you risk not getting an immediate update when the services changes and if it's too low you get flooded with events.

Start with the Consul Catalog Watch and the catalog services watch configuration described in the issue; the linked DiscoveryClientConfigServiceBootstrapConfiguration.java shows a current consumer. Trace how catalog changes become heartbeat events, then define how a service list would be configured and detected. Done means only updates to the selected services publish heartbeat events without flooding unrelated consumers.