dang, I'm in the same boat. Looks like the UI is crashing because of an invalid metric name after enabling Prometheus.
Dec 23 20:48:46 reef reef-pi[1199]: panic: descriptor Desc{fqName: "1_reading", help: "Module:1 Item:reading", constLabels: {}, variableLabels: []} is invalid: "1_reading" is not a valid metric name
Dec 23 20:48:46 reef reef-pi[1199]: goroutine 47 [running]:
Dec 23 20:48:46 reef reef-pi[1199]: github.com/prometheus/client_golang/prometheus.(*Registry).MustRegister(0x1d7b340, 0x1ee0cd8, 0x1, 0x1)
Dec 23 20:48:46 reef reef-pi[1199]: #011/home/ranjib/gocode/pkg/mod/github.com/prometheus/
[email protected]/prometheus/registry.go:401 +0x80
Dec 23 20:48:46 reef reef-pi[1199]: github.com/prometheus/client_golang/prometheus/promauto.Factory.NewGauge(0x7a4ed0, 0x1d7b340, 0x0, 0x0, 0x0, 0x0, 0x2098110, 0x9, 0x1efacc0, 0x15, ...)
Dec 23 20:48:46 reef reef-pi[1199]: #011/home/ranjib/gocode/pkg/mod/github.com/prometheus/
[email protected]/prometheus/promauto/auto.go:297 +0xc0
Dec 23 20:48:46 reef reef-pi[1199]: github.com/prometheus/client_golang/prometheus/promauto.NewGauge(...)
Dec 23 20:48:46 reef reef-pi[1199]: #011/home/ranjib/gocode/pkg/mod/github.com/prometheus/
[email protected]/prometheus/promauto/auto.go:191
Dec 23 20:48:46 reef reef-pi[1199]: github.com/reef-pi/reef-pi/controller/telemetry.(*telemetry).EmitMetric(0x1dc6000, 0xba7f11, 0x1, 0x6ac73a, 0x7, 0x28f5c28f, 0x40541f5c)
Dec 23 20:48:46 reef reef-pi[1199]: #011/home/ranjib/gocode/src/github.com/reef-pi/reef-pi/controller/telemetry/telemetry.go:194 +0x528
Dec 23 20:48:46 reef reef-pi[1199]: github.com/reef-pi/reef-pi/controller/modules/temperature.(*Controller).Check(0x1d8e960, 0x1db3500, 0x2, 0x0, 0x1, 0x0)
Dec 23 20:48:46 reef reef-pi[1199]: #011/home/ranjib/gocode/src/github.com/reef-pi/reef-pi/controller/modules/temperature/control.go:33 +0x430
Dec 23 20:48:46 reef reef-pi[1199]: github.com/reef-pi/reef-pi/controller/modules/temperature.(*Controller).Run(0x1d8e960, 0x1db3500, 0x20018c0, 0x0, 0x1)
Dec 23 20:48:46 reef reef-pi[1199]: #011/home/ranjib/gocode/src/github.com/reef-pi/reef-pi/controller/modules/temperature/tc.go:204 +0x130
Dec 23 20:48:46 reef reef-pi[1199]: created by github.com/reef-pi/reef-pi/controller/modules/temperature.(*Controller).Start
Dec 23 20:48:46 reef reef-pi[1199]: #011/home/ranjib/gocode/src/github.com/reef-pi/reef-pi/controller/modules/temperature/controller.go:91 +0x1c4
Dec 23 20:48:46 reef systemd[1]: reef-pi.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
Dec 23 20:48:46 reef systemd[1]: reef-pi.service: Failed with result 'exit-code'.