Add metrics API scaler docs

[turbaszek]

Related to kedacore/keda#929

[tomkerkhove]

Looks good overall but would like to follow the structure of other scalers and added some remarks.

[tomkerkhove]

And don't hesitate to drop a note about this new scaler in https://github.com/kedacore/keda-docs/blob/master/content/blog/keda-2.0-beta.md.draft#L26 given it allows for higher extensibility as well.

Happy to do for you if it's too much of a hassle.

[lonnix]

I looked over the docs and things look good. The only part I had to re-read was the bit about scaling down to 0 but it made sense after I read it a 2nd time.

[tomkerkhove]

I've added a bunch of suggestions to make it more readable/less tech detailed but it would be nice if you could align with the structure of all our scalers (example):

• Trigger Specification with parameter list after sample
• Authentication Parameters (none for now, but following https://keda.sh/docs/1.5/scalers/prometheus/#authentication-parameters)
• Example

How it works with the API could be a (sub)section before Authentication Parameters to talk about that or as part of the sample.

[turbaszek]

Thanks @tomkerkhove for your suggestions! I applied all of them

[tomkerkhove]

Almost there!

[tomkerkhove]

LGTM - Thanks @turbaszek!

[turbaszek]

Bold to emphasize than any API mean in fact any API, so one can scale their deployment using current day length from https://ipgeolocation.io/astronomy-api.html 😃