First, if this is the wrong place to post this, apologies, I didn’t know where to ask.
I use Cachet as a status page for a lot of my services. There is a plugin for Cachet called cachet-monitor that allows for HTTP and DNS monitoring with automatic status updates and a really nice way to hook into the built in metrics provided by Cachet.
On the repo’s README, in the contributions section, it mentions a TCP check. From what I can tell, no one has done this in any of the forks and the project is a bit stale. I don’t know Go, but thought some kind stranger on the internet with some free time could help out. I don’t think it would be too difficult to iterate off of the existing DNS or HTTP checks, but again I wouldn’t know where to start. There is one fork that seems to be a bit more maintained (here), but again, no TCP check.