fix cluster_helper.metrics_port zero means metric disable

[ElvisWai]

• Fix metrics_port = 0 仍然会开启端口  #647

• metrics_port 为0,监控不开启
• pprof_port 若不为0,将不会自增，出现抢占端口的问题
• 修复loop函数在metrics_port为0时报错，影响：监控不开启时，主进程无法通过日志确定当前同步进度

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[suxb201]

v3 版本不演进了，我暂时关掉这个 PR