fix(pd): set memberSize from RaftEngine members in IndexAPI

Author: caglareker

hugegraph-pd/hg-pd-service/src/main/java/org/apache/hugegraph/pd/rest/IndexAPI.java

@@ -66,6 +66,7 @@ public BriefStatistics index() throws PDException, ExecutionException, Interrupt
         BriefStatistics statistics = new BriefStatistics();
         statistics.leader = RaftEngine.getInstance().getLeaderGrpcAddress();
         statistics.state = pdService.getStoreNodeService().getClusterStats().getState().toString();
+        statistics.memberSize = RaftEngine.getInstance().getMembers().size();
         statistics.storeSize = pdService.getStoreNodeService().getActiveStores().size();
         statistics.graphSize = pdService.getPartitionService().getGraphs().size();
         statistics.partitionSize = pdService.getStoreNodeService().getShardGroups().size();