External DNS metric does not work on datadog

[tehlers320]

This https://github.com/DataDog/integrations-core/blob/7.68.0/external_dns/datadog_checks/external_dns/metrics.py#L9 is emitted as an epoch (gauge)

This is how everybody appears to use the metric

apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
  name: external-dns-cloudflare
spec:
  groups:
    - name: external-dns-cloudflare.rules
      rules:
        - alert: ExternalDNSCloudflareStale
          expr: |
            time() - external_dns_controller_last_sync_timestamp_seconds{job="external-dns-cloudflare"} > 60
          for: 5m
          annotations:
            summary: >-
              ExternalDNS ({{ $labels.job }}) has not synced successfully in the last five minutes
          labels:
            severity: critical

however datadog is not capable of using time()

[tehlers320]

#10428 @sarah-witt @mattjnewberry , how was this intended to be used once implemented?