status: expose metrics for error responses #19
Description
Airlock should count the number of error responses, on both API endpoints separately. Those two counters should be then exposed as Prometheus metrics via the status service.
After #17, each API has an associated machine-friendly "kind" (with bounded and low cardinality). Error kind can be used as a metric label to enhance observability.
Overall, final metrics should look like the following:
airlock_v1_pre_reboot_response_errors_total{kind="<XYZ>"}airlock_v1_steady_state_response_errors_total{kind="<XYZ>"}