I installed Uptime Kuma (https://github.com/louislam/uptime-kuma) on my dokku paas to monitor my dokku apps. It works great. It is great for pure HTTP services, but it can be used against things like RTMP servers because it also permits configuration of a health check with TCP pings. It gives me an email when things are down, and supports retry, heartbeat intervals, and can validate a string in the HTML retrieved. I love it.
I considerated this option but then realized that both sides, the api/services and the uptime checker will be in the same server then any problem impacting the server itself will leave offline the monitoring