New field request: cloud.instance.lifecycle

For Amazon EC2 instances, I like to record whether the instance lifecycle is `normal` or `spot`, in order to differentiate metrics between the two. Spot instances tend to be short(er) lived.

I'm currently specifying a field named `cloud.instance.lifecycle`. I'm not familiar with the equivalents in Azure or GCE, so I'm not sure if `lifecycle` is good generic term or not.