Add limit to alert messages.

[ahfuzhang]

1.add /metrics for this app;
2.add maxalertcount to avoid too many alerts;

If there are too many alerts, ding talk webhook api return like this:

{"caller":"dingtalk.go:103","component":"web","level":"error","msg":"Failed to send notification to DingTalk","respCode":460101,"respMsg":"description: body 大小不合法;solution:请保持大小在 20000bytes 以内;","target":"webhook1","ts":"2023-12-01T03:59:19.740Z"}