metrics assignment from variables

[shahzebsiddiqui]

we should be able to assign metrics from variables defined in tests. This could look as follows. We can have a property name vars which will look for key under the vars section of the test. If the key is found we will assign the value to the metric. In this example below we can perform an assertion equality.

buildspecs:
  metric_variables:
    executor: generic.local.bash
    type: script
    description: capture metrics from variables
    vars:
       FOO: BAR
    run: echo $FOO
    tags: tutorials
    metrics: 
      foo:
        type: str
        vars: foo 
    status:
      assert_eq:
        - name: foo
          ref: BAR

TODO:

• Update JSON Schema to add support for vars
• Add implementation for metrics with vars in https://github.com/buildtesters/buildtest/blob/devel/buildtest/builders/base.py
• Add example buildspec and regression test for metrics
• Add documentation in https://buildtest.readthedocs.io/en/devel/writing_buildspecs/performance_checks.html#defining-metrics.

[shahzebsiddiqui]

@szuananwar i will assign this task to you, this will require a bit of work