Lb complete lifecycle (#783)

* Add LoadBalancerLifecycle. Trigger lifecycle callbacks. Set hints from properties.

* Trigger LB lifecycle callbacks from BlockingLoadBalancerClient and RetryLoadBalancerInterceptor.

* Register LifecycleProcessors with @LoadBalancerClients

* Register LifecycleProcessors with @LoadBalancerClients configuration.

* Handle null lifecycle beans map returned from factory. Adjust tests to code changes.

* Handle null lifecycle beans map returned from factory in RetryLoadBalancerInterceptor. Ensure ReactiveLoadBalancer.Factory bean is present while instantiating RetryLoadBalancerInterceptor. Add more tests.

* Remove generics from supports(...) method signature.

* Allow setting hint per service via properties.

* Add some toString() methods. Add more info on deprecated callbacks in javadocs and comments.

* Add javadocs.

* Format javadocs. Add docs.

* Update hint docs.

* Fix docs.

* Fix docs.

* Extract filtering supported lifecycle processors to a separate class; Execute onComplete() calls for DISCARD status in RetryLoadBalancerInterceptor. Remove duplicated `onComplete` calls for FAILED and SUCCESS status in RetryLoadBalancerInterceptor. Add test for no duplicated lifecycle calls in RetryLoadBalancerInterceptorTest.

* Small refactoring: remove deprecated methods use, add final keywords, remove unnecessary keywords.

* Add javadoc.

docs/src/main/asciidoc/_configprops.adoc

@@ -34,6 +34,7 @@
 |spring.cloud.loadbalancer.health-check.initial-delay |  | 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.hint |  | Allows setting the value of <code>hint</code> that is passed on to the LoadBalancer request and can subsequently be used in {@link ReactiveLoadBalancer} implementations.
 |spring.cloud.loadbalancer.retry.enabled | true | 
 |spring.cloud.loadbalancer.ribbon.enabled | true | Causes `RibbonLoadBalancerClient` to be used by default.
 |spring.cloud.loadbalancer.service-discovery.timeout |  | String representation of Duration of the timeout for calls to service discovery.

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

@@ -917,8 +917,7 @@ in order to avoid retrying calls on a failing instance.
 `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.
+the value of the `spring.cloud.loadbalancer.health-check.path.default` property. 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]` property, substituting `[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.
 
@@ -947,6 +946,13 @@ 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`.
 
+[[spring-cloud-loadbalancer-hints]]
+=== Spring Cloud LoadBalancer Hints
+
+Spring Cloud LoadBalancer lets you set `String` hints that are passed to the LoadBalancer within the `Request` object and that can later be used in `ReactiveLoadBalancer` implementations that can handle them.
+
+You can set a default hint for all services by setting the value of the `spring.cloud.loadbalancer.hint.default` property. You can also set a specific value
+for any given service by setting the value of the `spring.cloud.loadbalancer.hint.[SERVICE_ID]` property, substituting `[SERVICE_ID]` with the correct ID of your service. If the hint is not set by the user, `default` is used.
 
 [[spring-cloud-loadbalancer-starter]]
 === Spring Cloud LoadBalancer Starter
@@ -1007,6 +1013,20 @@ public class MyConfiguration {
 ----
 ====
 
+[[loadbalancer-lifecycle]]
+=== Spring Cloud LoadBalancer Lifecycle
+
+One type of bean that it may be useful to register using <<custom-loadbalancer-configuration,Custom LoadBalancer configuration>> is `LoadBalancerLifecycle`.
+
+The LoadBalancerLifecycle beans provide callback methods, named `onStart(Request<RC> request)` and `onComplete(CompletionContext<RES, T> completionContext)`, that you should implement to specify what actions should take place before and after load-balancing.
+
+`onStart(Request<RC> request)` takes a `Request` object as a parameter. It contains data that is used to select an appropriate instance, including the downstream client request and <<spring-cloud-loadbalancer-hints,hint>>. On the other hand, a `CompletionContext` object is provided to the `onComplete(CompletionContext<RES, T> completionContext)` method. It contains the LoadBalancer `Response`, including the selected service instance, the `Status` of the request executed against that service instance and (if available) the response returned to the downstream client, and (if an exception has occurred) the corresponding `Throwable`.
+
+The `supports(Class requestContextClass, Class responseClass,
+Class serverTypeClass)` method can be used to determine whether the processor in question handles objects of provided types. If not overridden by the user, it returns `true`.
+
+NOTE: In the preceding method calls, `RC` means `RequestContext` type, `RES` means client response type, and `T` means returned server type.
+
 == Spring Cloud Circuit Breaker
 
 include::spring-cloud-circuitbreaker.adoc[leveloffset=+1]