Please see details on the attachment.
It depends a bit on what your users want, but I would probably do the following:
1) Create a line item with the formula: TAT Days = TRUE and ISNOTBLANK(Metric) (or include ISNOTBLANK(Metric in your TAT Days formula if it is not being used for anything else.
2) Filter on the new line item + your current filter on Metric Type
3) Set the "Show items that match any of the following:
You should try and combine the filter criteria into a single Boolean condition (Planual rule 4.02-01). This is much more efficient that separate filter criteria.
Also, filtering on nested dimensions is best avoided if possible (4.02-06), although obviously this depends on the requirement