No interface to `GSMetrics` in object wrapper

[simoncozens]

There's no documented interface from the Python GSFont object to get the metrics for a Glyphs3 font. I am having to do this:

metrics = Glyphs.font.metrics() # Method call, not property
metrics_names = [ m.name or m.typeName() for m in metrics]

which is fine, but super undocumented.

[schriftgestalt]

You would normally access them from the master, or even from the layer (the later would apply the filters, too).

[schriftgestalt]

I just added font.metrics and master.metrics to the wrapper. But only as readonly. There are several methods in objC that set those values but I need to find a way to expose them:
in GSFontMaster:

- (GSMetricValue *)setMetricPosition:(CGFloat)pos overshoot:(CGFloat)over type:(GSMetricsType)type name:(GSMetricsKey)name;