feat: make healthcheck interval configurable

ksql-rest-app/src/main/java/io/confluent/ksql/rest/server/KsqlRestConfig.java

@@ -71,6 +71,12 @@ public class KsqlRestConfig extends RestConfig {
       "Whether or not to set KsqlUncaughtExceptionHandler as the UncaughtExceptionHandler "
           + "for all threads in the application (this can be overridden). Default is false.";
 
+  public static final String KSQL_HEALTHCHECK_INTERVAL_MS_CONFIG =
+      KSQL_CONFIG_PREFIX + "healthcheck.interval.ms";
+  private static final String KSQL_HEALTHCHECK_INTERVAL_MS_DOC =
+      "Minimum time between consecutive healthcheck evaluations. Healthcheck queries before "
+          + "the interval has elapsed will receive cached responses.";
+
   private static final ConfigDef CONFIG_DEF;
 
   static {
@@ -102,14 +108,20 @@ public class KsqlRestConfig extends RestConfig {
         KSQL_SERVER_PRECONDITIONS,
         Type.LIST,
         "",
-       Importance.LOW,
-       KSQL_SERVER_PRECONDITIONS_DOC
+        Importance.LOW,
+        KSQL_SERVER_PRECONDITIONS_DOC
     ).define(
         KSQL_SERVER_ENABLE_UNCAUGHT_EXCEPTION_HANDLER,
         ConfigDef.Type.BOOLEAN,
         false,
         Importance.LOW,
         KSQL_SERVER_UNCAUGHT_EXCEPTION_HANDLER_DOC
+    ).define(
+        KSQL_HEALTHCHECK_INTERVAL_MS_CONFIG,
+        Type.LONG,
+        5000L,
+        Importance.LOW,
+        KSQL_HEALTHCHECK_INTERVAL_MS_DOC
     );
   }
 

ksql-rest-app/src/main/java/io/confluent/ksql/rest/server/resources/HealthcheckResource.java

@@ -75,7 +75,7 @@ public static HealthcheckResource create(
   ) {
     return new HealthcheckResource(
         new HealthcheckAgent(serviceContext, restConfig),
-        Duration.ofSeconds(15), // TODO: fetch from config
+        Duration.ofMillis(restConfig.getLong(KsqlRestConfig.KSQL_HEALTHCHECK_INTERVAL_MS_CONFIG)),
         System::currentTimeMillis
     );
   }