Add metadata metric to skyObjects

Metadata metrics to record the number of sky footprints successfully
added to source list.

python/lsst/meas/algorithms/skyObjects.py

@@ -191,6 +191,7 @@ def run(self, mask, seed, catalog=None):
         skyFootprints = generateSkyObjects(mask, seed, self.config)
         self.log.info("Added %d of %d requested sky sources (%.0f%%)", len(skyFootprints),
                       self.config.nSources, 100*len(skyFootprints)/self.config.nSources)
+        self.metadata['sky_footprint_count'] = len(skyFootprints)
 
         if skyFootprints and self.skySourceKey is not None and catalog is not None:
             for footprint in skyFootprints: