fix: nested top metrics sort on keyword field (#85058)

Using a double as a return value works only if the field we are
sorting on is a number. If the field is not a value we can convert
to a double, like a non-numeric keyword, converting it to a number
returns `NaN`. Without this patch, sorting takes place on the bucket
key, if the order field points to a non-numeric value. The additional
bucket key comparator is implicitly added as a tie breaker to avoid
non-deterministic sorting of buckets.

With this change we support sorting using any subclass of SortValue.
This means the bucket key will be used just in case of equal values
on the order field.

Issue: #78506