Adds ability for the TTL check to take into account the current application health status (#690)

Fixes gh-676

Co-authored-by: Spencer Gibb <spencer@gibb.us>

docs/src/main/asciidoc/_configprops.adoc

@@ -32,9 +32,11 @@
 |spring.cloud.consul.discovery.health-check-timeout |  | Timeout for health check (e.g. 10s).
 |spring.cloud.consul.discovery.health-check-tls-skip-verify |  | Skips certificate verification during service checks if true, otherwise runs certificate verification.
 |spring.cloud.consul.discovery.health-check-url |  | Custom health check url to override default.
+|spring.cloud.consul.discovery.heartbeat.actuator-health-group |  | The actuator health group to use (`null` for the root group) when determining system health via Actuator.
 |spring.cloud.consul.discovery.heartbeat.enabled | `+++false+++` | 
 |spring.cloud.consul.discovery.heartbeat.interval-ratio |  | 
 |spring.cloud.consul.discovery.heartbeat.reregister-service-on-failure | `+++false+++` | 
+|spring.cloud.consul.discovery.heartbeat.use-actuator-health | `true` | Whether or not to take the current system health (as reported via the Actuator Health endpoint) into account when reporting the application status to the Consul TTL check. Actuator Health endpoint also has to be available to the application.
 |spring.cloud.consul.discovery.heartbeat.ttl | `+++30s+++` | 
 |spring.cloud.consul.discovery.hostname |  | Hostname to use when accessing server.
 |spring.cloud.consul.discovery.include-hostname-in-instance-id | `+++false+++` | Whether hostname is included into the default instance id when registering service.
@@ -79,4 +81,4 @@
 |spring.cloud.consul.tls.key-store-password |  | Password to an external keystore.
 |spring.cloud.consul.tls.key-store-path |  | Path to an external keystore.
 
-|===
+|===

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

@@ -35,7 +35,7 @@ To activate Consul Service Discovery use the starter with group `org.springframe
 
 === Registering with Consul
 
-When a client registers with Consul, it provides meta-data about itself such as host and port, id, name and tags.  An HTTP https://www.consul.io/docs/agent/checks.html[Check] is created by default that Consul hits the `/actuator/health` endpoint every 10 seconds.  If the health check fails, the service instance is marked as critical.
+When a client registers with Consul, it provides meta-data about itself such as host and port, id, name and tags.  An https://www.consul.io/docs/discovery/checks#http-interval[HTTP Check] is created by default that Consul hits the `/actuator/health` endpoint every 10 seconds.  If the health check fails, the service instance is marked as critical.
 
 Example Consul client:
 
@@ -200,6 +200,78 @@ spring:
             - "Some other value"
 ----
 
+==== TTL Health Check
+
+A Consul https://www.consul.io/docs/discovery/checks#ttl[TTL Check] can be used instead of the default configured HTTP check.
+The main difference is that the application sends a heartbeat signal to the Consul agent rather than the Consul agent sending a request to the application.
+
+The interval the application uses to send the ping may also be configured. "10s" and "1m" represent 10 seconds and 1 minute respectively.
+The default is 30 seconds.
+
+This example illustrates the above (see the `spring.cloud.consul.discovery.heartbeat.*` properties in link:appendix.html[the appendix page] for more options).
+
+.application.yml
+----
+spring:
+  cloud:
+    consul:
+      discovery:
+        heartbeat:
+          enabled: true
+          ttl: 10s
+----
+
+===== TTL Application Status
+
+For a Spring Boot Actuator application the status is determined from its available health endpoint.
+When the health endpoint is not available (either disabled or not a Spring Boot Actuator application) it assumes the application is in good health.
+
+When querying the health endpoint, the root https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#production-ready-health-groups[health group] is used by default.
+A different health group can be used by setting the following property:
+
+.application.yml
+----
+spring:
+  cloud:
+    consul:
+      discovery:
+        heartbeat:
+          actuator-health-group: <your-custom-group-goes-here>
+----
+
+You can disable the use of the health endpoint entirely by setting the following property:
+
+.application.yml
+----
+spring:
+  cloud:
+    consul:
+      discovery:
+        heartbeat:
+          use-actuator-health: false
+----
+
+====== Custom TTL Application Status
+
+If you want to configure your own application status mechanism, simply implement the `ApplicationStatusProvider` interface
+
+.MyCustomApplicationStatusProvider.java
+----
+@Bean
+public class MyCustomApplicationStatusProvider implements ApplicationStatusProvider {
+	public CheckStatus currentStatus() {
+        return yourMethodToDetermineAppStatusGoesHere();
+    }
+}
+----
+and make it available to the application context:
+----
+@Bean
+public CustomApplicationStatusProvider customAppStatusProvider() {
+     return new MyCustomApplicationStatusProvider();
+}
+----
+
 ==== Actuator Health Indicator(s)
 If the service instance is a Spring Boot Actuator application, it may be provided the following Actuator health indicators.