Check and make sure that metric names is a list of strings

[dakinggg]

What does this PR do?

If you accidentally pass a string to Evaluator for metric_names, you get a semi nasty silent error of not actually filtering your metrics, because _filter_metrics will happily loop over the string you provided. This PR explicitly checks that the input of metric_names is a list.

What issue(s) does this change relate to?

Closes CO-1505

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[mvpatel2000]

minor nits