Use labels for mapping metric types to metrics

[mikkeloscar]

For historical reasons (i.e. use of previous autoscaling API versions) we have been mapping metric names to the different collector types. I.e. if you wanted a metric based on prometheus you would name the metric prometheus-query.
This design has the downside that multiple metrics will have the same name and it becomes hard to distinguish as reported in #104
In some cases it can even prevent having more than a single metric of a certain type i.e. if you use the zmon-check metric and have the same check-id but different complex keys defined (complex keys must be defined as annotation, and this will overlap if you have more than a single zmon-check metric defined).

It was initially done because we didn't have any other identifiers in the HPA spec, at least for custom metrics, and this limitation was accidentally carried over for External metrics even though they have labels which can be used for such extra information. With v2beta2 we have selector/labels for all the metric types and this can then be used to store this information.

This PR introduces support for multiple metrics of the same type by adding a convention of a type label on the metrics e.g. type: prometheus which is used to identify which collector type should be used.

I have also aligned some of the naming for the types so they are more simple i.e. prometheus-query becomes type prometheus.

To provide backwards compatibility the old metric names are still supported, but marked as legacy so we eventually can deprecate and remove them.

Fix #104

[szuecs]

@mikkeloscar can we log a warning in case old style is used?
This would help operators to migrate safely.

[mikkeloscar]

@szuecs added a warning log

[szuecs]

👍

[mikkeloscar]

Found a bug where the fallback didn't work in case some labels were defined for the metric. Fixed and added tests to cover this.

[mikkeloscar]

👍

[csenol]

👍

[szuecs]

👍