Add support for health indicator groups

[ngaya-ll]

My organization uses Spring Boot services running in a Kubernetes environment. Kubernetes has a concept of two types of health probes: liveness and readiness probes. Liveness probes are used to detect if the service needs to be restarted, while readiness probes detect if the service is ready to be added to load balancers.

It would be great if there were a way to perform different checks for readiness and liveness. For example, if an application temporarily loses connectivity to its data source it should be taken out of load balancing, but doesn't need to be restarted. So we would like to include the DataSourceHealthIndicator in the readiness probe but not the liveness probe.

Note that #8685 already allows querying a single health indicator, but not multiple indicators. Perhaps Spring Boot could provide a way to configure named groups of indicators that could be accessed as /actuator/health/{group}.

[eddumelendez]

I am working in this issue. I will be sending a PR this weekend.

[wilkinsona]

Thank you, @eddumelendez. We had a few thoughts when we discussed this earlier this week. They are only initial thoughts so they aren't prescriptive, but hopefully they'll be of some use.

We envisaged a property-based mechanism for creating a group of health indicators. In application.properties, the value of the property would probably be a comma-separated list of the ids of the health indicators that are to be included in the group. The key would be the name of the group.

We then thought that CompositeHealthIndicator could be reused and created with all the health indicators in the group. One gotcha that @philwebb identified is that these composite health indicators that represent a group will need to be hidden from the main health check otherwise any grouped indicators would be called twice for a single check.

[eddumelendez]

Thanks for letting me know @wilkinsona I'll be working on this taking into account these thoughts.