External-dns: Feature Request: Prometheus metrics on sync status

[Evesy]

Hi,

I was hoping to be able to alert when a record is failing to be sync'd by external DNS (e.g. due to invalid cloud credential etc.) and though it'd be nice if external dns exposed some additional metrics around this, i.e

Sync'd:

external_dns_record_synced{domain="mydomain.company.com",provider="clouddns",ttl="3600} 1

Not sync'd:

external_dns_record_synced{domain="mydomain.company.com",provider="clouddns",ttl="3600} 0

I saw some other metrics were proposed in #179 but never merged in.

Not familiar with golang but would be happy to give it a try if it's fairly trivial & anyone has some pointers, but either way I'd love to see this

[Raffo]

Please feel free to open a PR when you have this ready, we are happy to review and help!