onionprobe's exporter port allocation conflict with the push gateway
We luckily don't have the push gateway enabled on the monitoring server, but we might eventually use it! The push gateway listens on port 9091, and this overlaps with the port chosen by onionprobe for its exporter.
The "writing a new exporter" guide:
https://prometheus.io/docs/instrumenting/writing_exporters/#port-numbers
... explicitly mentions that you need to register your port in this wiki page:
https://github.com/prometheus/prometheus/wiki/Default-port-allocations
do so, then change the port number here.