check here

1. Enable REST API on MySQL Router here
2. Download binary from release page and put to /usr/local/bin
3. Edit mackerel-agent.conf then restart.

[plugin.metrics.mysqlrouter]
command = ["/usr/local/bin/mackerel-plugin-mysqlrouter"]
env = { MYSQLROUTER_URL = "http://localhost:8080", MYSQLROUTER_USER = "luis", MYSQLROUTER_PASS = "luis", MYSQLROUTER_TLS_SKIP_VERIFY = "true" }

You should set alert of health per route like below.

This 0.1 is no meaning. If route is no heath return 0.

• Show graph per route connections