Replace calls to `BSONElement::fieldName()` with calls to `BSONElement::fieldNameStringData()` in cases where the knowing the field name length would be beneficial.
`BSONElement::fieldName()` only returns a const char* pointer, whereas `fieldNameStringData()` returns an std::string_view with an already pre-calculated length.
Stuffing the return value from `BSONElement::fieldName()` into other std::string_views requires recalculating the field name length, which is totally avoidable by using `fieldNameStringData()` instead.