Metrics for Onion Services in Arti

As part of Project 161 we should add various metrics needed for Onion Service operators to keep track and monitor the behaviour of their Onion Services. This needs coordination with our MetricsPort deliverables under Project 141

Activities like the following that will evolve over time:

• Expose more prometheus metrics for operators
• Provide grafana dashboards that clearly show how metrics are changing over time
• Expose onionperf data on metrics.tpo