code: public CompletableFuture<Integer> getUserSize() {
          if (this.localEngine != null && this.sncpNodeAddresses == null) {
              return CompletableFuture.completedFuture(this.localEngine.getLocalUserSize());
          }
          tryAcquireSemaphore();
          CompletableFuture<Integer> rs = this.sncpNodeAddresses.queryKeysStartsWithAsync(SOURCE_SNCP_USERID_PREFIX).thenApply(v -> v.size());
          if (semaphore != null) rs.whenComplete((r, e) -> releaseSemaphore());
          return rs;
      }
docs: 获取在线用户总数
  
  
  @return boolean