A plugin fails to start when a task references metrics from two versions of the same plugin.

[jcooklin]

Given two different versions of the same plugin where the first plugin contains a metric that is no longer exposed in version 2 the first plugin will not be started and the task fails.

To recreate the scenario:

• Start snap with the following config

---
control:
  plugins:
    collector:
      mock:
        versions:
          1:
            test: true

• Load the mock plugins (versions 1 and 2)
• Create and start the following task

---
  version: 1
  schedule:
    type: "simple"
    interval: "1s"
  workflow:
    collect:
      metrics:
        /intel/*: {}
      config:
        /intel/mock:
          user: "root"
          password: "secret"