Fix: Create separate metrics for HSDir info and timestamp of last update (#52)

Two breaking changes in metric collection:

1. Removed the updated_at label from all metrics, which was creating a new data series for every measurement.

2. Removed the hsdir label from onion_service_descriptor_reachable metric, which was creating a new data series for every measurement.

Adds two new metrics:

1. onion_service_descriptor, with Onion Service descriptor information.
2. onion_service_probe_status, with timestamp from the last test.