external-dns: Add servicemonitor metrics label

[gracedo]

What type of PR is this?

Feature

What this PR does/ why we need it:

Enables metrics collection by adding the servicemonitor label

Which issue(s) this PR fixes:

https://jira.d2iq.com/browse/D2IQ-72698

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

external-dns: Add servicemonitor label to enable metrics collection by Prometheus

Checklist

• The commit message explains the changes and why are needed.
• The code builds and passes lint/style checks locally.
• The relevant subset of integration tests pass locally.
• The core changes are covered by tests.
• The documentation is updated where needed.

[gracedo]

Tested manually on a cluster

[samvantran]

had a question but not blocking since your manual tests show it works 👍

[samvantran]

you don't need to add a name like for other fixes? eg like this

[gracedo]

for external-dns, we can reuse an existing servicemonitor that just selects on that one label (https://github.com/mesosphere/kubernetes-base-addons/blob/master/addons/prometheus/prometheus.yaml#L83-L86)