Allow enabling cloudwatch auto scaling group metrics collection for HA mode

[denvazh]

For more serious use and use in production environment I found that I would like to have some way to setup alerts.
It seems Auto Scaling group by default has corresponding metrics collection disabled and its up to the user to either enable allow of them or enable specific ones.

This change allows specifying ( and thus enabling collection ) for specific Auto Scaling group metrics.

Group metrics are available at one-minute granularity at no additional charge, but you must enable them.

Docs: https://docs.aws.amazon.com/autoscaling/ec2/userguide/ec2-auto-scaling-metrics.html

Example when changes are applied and collection of GroupInServiceInstances metric was enabled.

[RaJiska]

Thank you @denvazh , this is a great addition.

Considering enabling those metrics is free as you pointed out, I believe those should be enabled by default as there is no downside to having them. I'd even say it could be great if a couple more could be enabled, such as:

• GroupInServiceInstances,
• GroupPendingInstances,
• GroupStandbyInstances, and,
• GroupTerminatingInstances

[denvazh]

@RaJiska Sure! Let me adjust the code to reflect that idea and have metrics you've mentioned enabled by default 👍

[RaJiska]

Completed through #24, thank you @denvazh.