Adapt external-dns to only use required informers depending on filters and add support for multi-namespace watch

[ecojan]

What would you like to be added: There are 2 proposed changes in this issue, will try and elaborate both:

1. Adapt external-dns to only use required informers depending on filters (ability to run in a reduced scope) Current form of external-dns assumes it would always be run as a deployment with cluster wide access requesting information on resources when it might not even need them. One such example is the Services source, where, no matter what type of service one might filter one (example only using Service type external), external-dns would still request the node-informer. This could easily be solved by checking the filter types and only initialising the nodeInformer when needed. The end result would be that one could start using external-dns in a namespaced scenarios, albeit while only having access to certain type of resources.

2. Add support for multi-namespace watch Due to limitations imposed by the K8s client/server libraries, we cannot watch multiple specific namespaces with 1 informer (we can only watch either one namespace or all). External-dns already accepts a namespace argument. One possible solution to this would be to expand this to accept multiple namespaces (separated by ,) by using a structure of <namespace, informers> and saving it as map that could be then used to instantiate the informers for all of the specified namespaces.

Why is this needed:

Making external-dns to be runnable by a user in a namespaced scenario (namespace admin) where one would need to watch multiple namespaces.

[k8s-triage-robot]

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/lifecycle stale

[ecojan]

/remove-lifecycle stale

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/lifecycle stale

[ecojan]

/remove-lifecycle stale

[k8s-triage-robot]

The Kubernetes project currently lacks enough contributors to adequately respond to all issues and PRs.

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• After 90d of inactivity, lifecycle/stale is applied
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/lifecycle stale

[ecojan]

/remove-lifecycle stale

[k8s-triage-robot]

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This bot triages un-triaged issues according to the following rules:

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/lifecycle stale

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/lifecycle rotten

[ecojan]

/remove-lifecycle stale

[ecojan]

/remove-lifecycle rotten

[k8s-triage-robot]

The Kubernetes project currently lacks enough contributors to adequately respond to all issues.

This bot triages un-triaged issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
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You can:

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Please send feedback to sig-contributor-experience at kubernetes/community.

/lifecycle stale

[ecojan]

/remove-lifecycle stale

[k8s-triage-robot]

The Kubernetes project currently lacks enough contributors to adequately respond to all issues.

This bot triages un-triaged issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
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You can:

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/lifecycle stale

[k8s-triage-robot]

The Kubernetes project currently lacks enough active contributors to adequately respond to all issues.

This bot triages un-triaged issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
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/lifecycle rotten

[k8s-triage-robot]

The Kubernetes project currently lacks enough active contributors to adequately respond to all issues and PRs.

This bot triages issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
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/close not-planned

[k8s-ci-robot]

@k8s-triage-robot: Closing this issue, marking it as "Not Planned".

In response to this:

The Kubernetes project currently lacks enough active contributors to adequately respond to all issues and PRs.

This bot triages issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
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/close not-planned

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