Get overall metric information for a particular font description.
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.
The `PangoFontDescription` is interpreted in the same way as by [func@itemize], and the family name may be a comma separated list of names. If characters from multiple of these families would be used to render the string, then the returned fonts would be a composite of the metrics for the fonts loaded for the individual families.
a `PangoFontDescription` structure. null means that the font description from the context will be used.
language tag used to determine which script to get the metrics for. null means that the language tag from the context will be used. If no language tag is set on the context, metrics for the default language (as determined by [ func@Pango.Language.get_default] will be returned.
a `PangoFontMetrics` object. The caller must call [method@Pango.FontMetrics.unref] when finished using the object.