Allow refetching instances for healthcheck (#855)

* Allow refetching instances by HealthCheckServiceInstanceListSupplier.

* Add docs and javadocs.

* Fix docs after review.

docs/src/main/asciidoc/_configprops.adoc

@@ -10,10 +10,6 @@
 |spring.cloud.discovery.client.health-indicator.enabled | true | 
 |spring.cloud.discovery.client.health-indicator.include-description | false | 
 |spring.cloud.discovery.client.simple.instances |  | 
-|spring.cloud.discovery.client.simple.local.instance-id |  | The unique identifier or name for the service instance.
-|spring.cloud.discovery.client.simple.local.metadata |  | Metadata for the service instance. Can be used by discovery clients to modify their behaviour per instance, e.g. when load balancing.
-|spring.cloud.discovery.client.simple.local.service-id |  | The identifier or name for the service. Multiple instances might share the same service ID.
-|spring.cloud.discovery.client.simple.local.uri |  | The URI of the service instance. Will be parsed to extract the scheme, host, and port.
 |spring.cloud.discovery.client.simple.order |  | 
 |spring.cloud.discovery.enabled | true | Enables discovery client health indicators.
 |spring.cloud.features.enabled | true | Enables the features endpoint.
@@ -34,6 +30,9 @@
 |spring.cloud.loadbalancer.health-check.initial-delay | 0 | Initial delay value for the HealthCheck scheduler.
 |spring.cloud.loadbalancer.health-check.interval | 25s | Interval for rerunning the HealthCheck scheduler.
 |spring.cloud.loadbalancer.health-check.path |  | 
+|spring.cloud.loadbalancer.health-check.refetch-instances | false | Indicates whether the instances should be refetched by the <code>HealthCheckServiceInstanceListSupplier</code>. This can be used if the instances can be updated and the underlying delegate does not provide an ongoing flux.
+|spring.cloud.loadbalancer.health-check.refetch-instances-interval | 25s | Interval for refetching available service instances.
+|spring.cloud.loadbalancer.health-check.repeat-health-check | true | Indicates whether health checks should keep repeating. It might be useful to set it to <code>false</code> if periodically refetching the instances, as every refetch will also trigger a healthcheck.
 |spring.cloud.loadbalancer.retry.enabled | true | 
 |spring.cloud.loadbalancer.retry.max-retries-on-next-service-instance | 1 | Number of retries to be executed on the next <code>ServiceInstance</code>. A <code>ServiceInstance</code> is chosen before each retry call.
 |spring.cloud.loadbalancer.retry.max-retries-on-same-service-instance | 0 | Number of retries to be executed on the same <code>ServiceInstance</code>.

docs/src/main/asciidoc/spring-cloud-commons.adoc

@@ -953,16 +953,22 @@ TIP: This mechanism is particularly helpful while using the `SimpleDiscoveryClie
 clients backed by an actual Service Registry, it's not necessary to use, as we already get
 healthy instances after querying the external ServiceDiscovery.
 
-TIP:: This supplier is also recommended for setups with a small number of instances per service
+TIP: This supplier is also recommended for setups with a small number of instances per service
 in order to avoid retrying calls on a failing instance.
 
+WARNING: If using any of the Service Discovery-backed suppliers, adding this health-check mechanism is usually not necessary, as we retrieve the health state of the instances directly
+from the Service Registry.
+
+TIP: The `HealthCheckServiceInstanceListSupplier` relies on having updated instances provided by a delegate flux. In the rare cases when you want to use a delegate that does not refresh the instances, even though the list of instances may change (such as the `ReactiveDiscoveryClientServiceInstanceListSupplier` provided by us), you can set `spring.cloud.loadbalancer.health-check.refetch-instances` to `true` to have the instance list refreshed by the `HealthCheckServiceInstanceListSupplier`. You can then also adjust the refretch intervals by modifying the value of `spring.cloud.loadbalancer.health-check.refetch-instances-interval` and opt to disable the additional healthcheck repetitions by setting `spring.cloud.loadbalancer.repeat-health-check` to `fasle` as every instances refetch
+  will also trigger a healthcheck.
+
 `HealthCheckServiceInstanceListSupplier` uses properties prefixed with
 `spring.cloud.loadbalancer.health-check`. You can set the `initialDelay` and `interval`
 for the scheduler. You can set the default path for the healthcheck URL by setting
 the value of the `spring.cloud.loadbalancer.health-check.path.default`. You can also set a specific value
 for any given service by setting the value of the `spring.cloud.loadbalancer.health-check.path.[SERVICE_ID]`, substituting the `[SERVICE_ID]` with the correct ID of your service. If the path is not set, `/actuator/health` is used by default.
 
-TIP:: If you rely on the default path (`/actuator/health`), make sure you add `spring-boot-starter-actuator` to your collaborator's dependencies, unless you are planning to add such an endpoint on your own.
+TIP: If you rely on the default path (`/actuator/health`), make sure you add `spring-boot-starter-actuator` to your collaborator's dependencies, unless you are planning to add such an endpoint on your own.
 
 In order to use the health-check scheduler approach, you will have to instantiate a `HealthCheckServiceInstanceListSupplier` bean in a <<custom-loadbalancer-configuration,custom configuration>>.
 
@@ -987,7 +993,7 @@ public class CustomLoadBalancerConfiguration {
 	}
 ----
 
-NOTE:: `HealthCheckServiceInstanceListSupplier` has its own caching mechanism based on Reactor Flux `replay()`, therefore, if it's being used, you may want to skip wrapping that supplier with `CachingServiceInstanceListSupplier`.
+WARNING: `HealthCheckServiceInstanceListSupplier` has its own caching mechanism based on Reactor Flux `replay()`. Therefore, if it's being used, you may want to skip wrapping that supplier with `CachingServiceInstanceListSupplier`.
 
 
 [[spring-cloud-loadbalancer-starter]]