Provide detailed tutorial / documentation on HPAv2

[alexellis]

Description

We should provide detailed tutorial / documentation on HPAv2 when deploying OpenFaaS to Kubernetes using faas-netes. This would include any caveats or work-in-progress around scale to/from-zero.

Please respond here with an outline of the tutorial / guide and then send a PR to https://github.com/openfaas/docs/ under the heading you feel is best suited. Instructions should be tested and repeatable.

Context

HPAv2 may provide a suitable alternative for some use-cases.

Whilst it has been used before, we have not documented it thoroughly for new users to Kubernetes and this would lower the barrier significantly.

Ref: openfaas/faas#1271

[alexellis]

For HPAv2 to work, it's compulsory to install the "metrics server":

https://github.com/kubernetes-incubator/metrics-server

Note that the metrics server is not supported on all cloud platforms, several like DigitalOcean currently need an insecure TLS option to be overridden to operate.

@stefanprodan has a tutorial about HPA with Prometheus custom metrics, I would think some of the content is useful for whoever makes this new tutorial, but chunked up to a much higher level of detail.

https://github.com/stefanprodan/k8s-prom-hpa