Gets overall metric information for a font.
Since the metrics may be substantially different for different scripts, a language tag can be provided to indicate that the metrics should be retrieved that correspond to the script(s) used by that language.
NULL, this function gracefully sets some sane values in the
output variables and returns.
Language tag used to determine which script to get the metrics for, or
NULLto indicate to get the metrics for the entire font.
The argument can be
The data is owned by the caller of the method.