STRING metrics in some VCFs can actually be numbers (just mislabelled)

Currently ignoring all metrics except INT or FLOAT but sometimes metrics can be labelled as STRING or anything else despite being digits (and therefore plottable)

Should check metric values(?) only contain numeric digits and go from there when grabbing available metrics

Start by tracing how VCF metrics are discovered and how the current INT/FLOAT filter feeds the plotting path. Check the existing metric-selection logic and its tests, if present. Done means digit-only values from STRING-labelled metrics are available for plotting while nonnumeric metrics remain excluded.

Written by the indexing model from the issue text.