Mechanism for serialising metric requirements

[lewisjared]

The problem

The metric packages may not be able to be installed in the same virtual environment as the ref. We need a mechanism of creating proxy metrics/providers when we can't import the instance of the metric directly.

Perhaps we need a mechanism to serialise the provider and metric definitions. Probably YAML formatted. This could be added to a pre-commit hook to ensure that the definitions and the serialised output remain in sync.

The file would then be checked into the repo.

It should include version information about the provider

Definition of "done"

• [ ] Add cattrs or some other way of serialising a provider and it's metrics
• [ ] Add a CLI endpoint to serialise the results
• [ ] Add a proxy metric (same metadata, but a dummy run function)
• [ ] Build a provider from this metric information

Additional context

CMEC does something similar with it's metric definitions

[lewisjared]

This might be more difficult with Constraints. I don't think we can support arbitrary functions instead we must use a set of constraints provided by ref-core