Review and improve Prometheus rules and tests

Description

Improvements for the "Onionprove not responding" alert were proposed at tpo/tpa/prometheus-alerts#27 (closed). This ticket is about evaluating, integrating and improving the suggestions.

Tasks

• Add a new, simpler alert for when Onionprobe is down.
• Test solution for tpo/tpa/prometheus-alerts#27 (closed), especially when there are no samples/no series.
• Decide whether this new test should be integrated, and improve it if needed.

Time estimation

• Complexity: very small (0.5 day)
• Uncertainty: low (x1.1)
• Reference (adapted)