Sets the metrics hinting mode for the font options object. This
controls whether metrics are quantized to integer values in
device units.
The following constants can be passed in the hint_metrics argument:
#CAIRO_HINT_METRICS_DEFAULT-
Hint metrics in the default manner for the font backend and target device.
#CAIRO_HINT_METRICS_OFF-
Do not hint font metrics.
#CAIRO_HINT_METRICS_ON-
Hint font metrics.