fix issue that server list never updated

apm-sniffer/apm-agent-core/src/main/java/org/skywalking/apm/agent/core/remote/DiscoveryRestServiceClient.java

@@ -5,6 +5,7 @@ import com.google.gson.JsonArray;
 import com.google.gson.JsonElement;
 import java.io.IOException;
 import java.util.LinkedList;
+import java.util.List;
 import java.util.Random;
 import org.apache.http.client.methods.CloseableHttpResponse;
 import org.apache.http.client.methods.HttpGet;
@@ -66,11 +67,14 @@ public class DiscoveryRestServiceClient implements Runnable {
                         for (JsonElement element : serverList) {
                             newServerList.add(element.getAsString());
                         }
-                        if (!newServerList.equals(GRPC_SERVERS)) {
+
+                        if (!isListEquals(newServerList, GRPC_SERVERS)) {
+                            GRPC_SERVERS = newServerList;
                             logger.debug("Refresh GRPC server list: {}", GRPC_SERVERS);
                         } else {
                             logger.debug("GRPC server list remain unchanged: {}", GRPC_SERVERS);
                         }
+
                     }
                 }
             }
@@ -82,6 +86,20 @@ public class DiscoveryRestServiceClient implements Runnable {
         }
     }
 
+    private boolean isListEquals(List<String> list1, List<String> list2) {
+        if (list1.size() != list2.size()) {
+            return false;
+        }
+
+        for (String ip1 : list1) {
+            if (!list2.contains(ip1)) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
     /**
      * Prepare the given message for HTTP Post service.
      *