Allow multiple provider specific annotations of same type

[roehrijn]

Description

External DNS already supports exposing a single K8s service behind multiple hostnames by using the external-dns.alpha.kubernetes.io/hostname annotation with a comma-seperated list of hostnames in its value. One would do this for example for making a service availible with a technical, static hostname and a DNS load-balancer, like Route53 can act as, at the same time.
However, when defining additonal annotations on the service, e.g. external-dns.alpha.kubernetes.io/aws-weight this always applies to all the hostnames of the service. This makes it impossible to define different weights for different hostnames on the same service. At the same time it is not possible that an annotation only applies to a single one and not all the hostnames of a service.

This PR implements a solution to make exactly the described configuration possible by introducing a kind of "string map" notation in the annotations' values.

For example:

external-dns.alpha.kubernetes.io/hostname: my.foo.app, my.foo.internal
external-dns.alpha.kubernetes.io/aws-weight: my.foo.app=100

would define the two hostname my.foo.app and my.foo.internal, whereas a load balancing weight is only set for my.foo.app.

Checklist

• Unit tests updated
• End user documentation updated

_{Jan Roehrich [email protected], Mercedes-Benz Tech Innovation GmbH, legal info/Impressum}

[k8s-ci-robot]

Hi @roehrijn. Thanks for your PR.

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[mloiseleur]

/ok-to-test

[mloiseleur]

Thanks. It looks good, implementation is light and it's documented 👍
/lgtm
/assign @Raffo

[ivankatliarchuk]

Looks good. In general we should have a file annotation_processor.go or something where all the related logic exist. As at the moment all annotation processing is all over the codebase, which makes it difficult to consume.

/lgtm
cc: @mloiseleur

[ivankatliarchuk]

/ok-to-test

[ivankatliarchuk]

Before we hit the merge button @roehrijn Could you also provide a set of manual test steps using manifests and kubectl commands? This will help in understanding and verifying the implementation.

The code itself looks good.

[k8s-ci-robot]

PR needs rebase.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[ivankatliarchuk]

/lgtm cancel

needs rebase and some testing fixtures to validate the change

[ivankatliarchuk]

Hi @roehrijn are you still interesting in merging this pull request?

[ivankatliarchuk]

/remove-approve

[k8s-ci-robot]

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[ivankatliarchuk]

/label tide/merge-method-squash

[ivankatliarchuk]

/lgtm cancel