This is a very simple prometheus metrics target that can be scraped.
After start it let's the user set a new value for the gauge that it exposes for prometheus.
Just use
go run promtestsource.go
or
go run promtestsource.go :5002
to set a different port and run multiple instances.
Fetch the current metrics manually with
curl http://localhost:5001/metrics
Simple way to trigger alerts and test them.