Enhance EtcdCoordinator if disconnected from etcd server for a long time (#3782)

* Update EtcdCoordinator.java

when EtcdCoordinator cannot connect to etcd cluster for a long time, 
the information registered in etcd will expire and refresh() function will not work.

oap-server/server-cluster-plugin/cluster-etcd-plugin/src/main/java/org/apache/skywalking/oap/server/cluster/plugin/etcd/EtcdCoordinator.java

@@ -34,11 +34,14 @@ import org.apache.skywalking.oap.server.core.cluster.RemoteInstance;
 import org.apache.skywalking.oap.server.core.cluster.ServiceRegisterException;
 import org.apache.skywalking.oap.server.core.remote.client.Address;
 import org.apache.skywalking.oap.server.telemetry.api.TelemetryRelatedContext;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * @author Alan Lau
  */
 public class EtcdCoordinator implements ClusterRegister, ClusterNodesQuery {
+    private static final Logger logger = LoggerFactory.getLogger(EtcdCoordinator.class);
 
     private ClusterModuleEtcdConfig config;
 
@@ -113,7 +116,11 @@ public class EtcdCoordinator implements ClusterRegister, ClusterNodesQuery {
             try {
                 client.refresh(key, KEY_TTL).send().get();
             } catch (Exception e) {
-
+                try {
+                    client.put(key, json).ttl(KEY_TTL).send().get();
+                } catch (Exception ee) {
+                    logger.error(ee.getMessage(), ee);
+                }
             }
         }, 5 * 1000, 30 * 1000, TimeUnit.MILLISECONDS);
     }