pass-through strategy plugin

[jazzyfresh]

This feature would introduce a new strategy plugin for simple pass-through scaling action. That is, whatever the value returned by the APM, this strategy will scale to that value exactly.

strategy "pass-through" {}

This strategy would be incorporated into a scaling policy like so:

scaling "batch_processing_cluster" {
  min = 0
  max = 20

  policy {
    check "queue_size" {
      source = "prometheus"
      query  = "running_jobs + pending_jobs"

      strategy "pass-through" {}
    }

    target "aws-asg" {
      aws_asg_name = "batch_clients"
      node_class = "high_memory"
    }
  }
}

[jazzyfresh]

TODO: add documentation to nomadproject.io website in nomad repo

[lgfa29]

Early review and it's looking good! Just a few places with the wrong plugin name.

[cgbaker]

looks good to me!
why pass-thru and not pass-through?

[lgfa29]

I pushed a commit to guard against empty metric results. It shouldn't really happen inside a plugin (the worker won't call the plugin without metrics), but just in case.

This check is also present in the target-value plugin, but it's wrong 😬