support accessing all counters within JobTest

[catherinejelder]

I would like to be able to access counters within in a JobTest without knowing their names in advance, maybe with a .counters method that returns all counters.

[nevillelyh]

Should be easy to impl. It's a matter of adding methods similar to counter/distribution/gauge here that take an assertion fn and calling it in the metricsFn here.

[rulle-io]

@catherinejelder Can you, please, provide an example code snippet, showing how do you want to access counters within in a JobTest.