Add metrics for secondary DNS one transfer failure.

[swapmat-f5]

• Program: Authoritative
• Issue type: Feature request

Short description

Add metrics for secondary DNS one transfer success and failure.

Usecase

Export zone transfer failure metrics for alerting.

Description

Based on the xfr-cycle-interval, powerdns performs zone transfer but this can fail. There are logs present to capture the error but adding metrics should also be considered.
These metrics should be available on the /metrics API endpoint.
This will help in use-cases like -
prometheus based alerts to be raised if the number of xfr failures are beyond some threshold, etc.