Add metrics-server addon

[itskingori]

From the official core metrics pipeline docs:

Starting from Kubernetes 1.8, resource usage metrics, such as container CPU and memory usage, are available in Kubernetes through the Metrics API. These metrics can be either accessed directly by user, for example by using kubectl top command, or used by a controller in the cluster, e.g. Horizontal Pod Autoscaler, to make decisions.

The metrics server is not available in a Kubernetes cluster (unless it's been brought up by kube-up.sh and that's only 1.8+). So it's good to have it here. This complements HPA documentation that will be added by #3942.

See kubernetes-sigs/metrics-server#32 also. It's not necessary for end user but useful for context around the compatibility matrix.

[itskingori]

/assign @justinsb

[robinpercy]

@justinsb @chrislovecnm any concern with using directories for addon management vs single manifests?

[itskingori]

@robinpercy Oh, I see. Actually, I think single file would be best so that people aren't forced to clone the repo or somehow copy the folder. One file would be easier as you can kubectl apply the raw github file.

[itskingori]

@robinpercy @chrislovecnm changed to single file in ae032eb.

[robinpercy]

/assign

Thanks @itskingori! Looks, good. Just have the one question for Justin or Chris around the directory structure.

[chrislovecnm]

@robinpercy We need the addons file and it needs to be tested with channels. I am guessing that we need a single manifest, but again we need to test using the channel binary.

[itskingori]

@chrislovecnm about #4581 (comment), do you have a reference PR I can look at to know how to do that?

[chrislovecnm]

@itskingori how to test or the file layout?

[itskingori]

@chrislovecnm both, how to register it as an addon via the file (assuming that's what you meant) ... and to test.

[chrislovecnm]

@itskingori https://github.com/kubernetes/kops/tree/master/addons/monitoring-standalone has a good layout of files, and you can build channels and I think it is channel apply -f addon.yaml ... I think. Protokube runs it all the time and admitted I have rarely run it by hand. We need the addon.yaml file and I think we want the addon in just one YAML manifest, but you may be able to mess with the addon.yaml file and use multiple. We do not have an addon directory with multiple directories in it, so I am hesitant to have one with subfolders.

Ping me or #kops-dev if you have more questions.

[justinsb]

Awesome - thank you @itskingori

I'd like to get this in. We can also look at integrating it, but we're starting to lay the groundwork for adding arbitrary addons via a manifest (see AddonSpec at https://github.com/kubernetes/kops/blob/master/pkg/apis/kops/cluster.go#L51) so this might be a great way to install in that way!

[justinsb]

/lgtm

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[justinsb]

/ok-to-test