[artstation] add 'num' and 'count' metadata fields (#2764)

gallery_dl/extractor/artstation.py

@@ -32,9 +32,11 @@ class ArtstationExtractor(Extractor):
         data = self.metadata()
 
         for project in self.projects():
-            for asset in self.get_project_assets(project["hash_id"]):
+            for num, asset in enumerate(
+                    self.get_project_assets(project["hash_id"]), 1):
                 asset.update(data)
                 adict = asset["asset"]
+                asset["num"] = num
                 yield Message.Directory, asset
 
                 if adict["has_embedded_player"] and self.external:
@@ -85,6 +87,7 @@ class ArtstationExtractor(Extractor):
         assets = data["assets"]
         del data["assets"]
 
+        data["count"] = len(assets)
         if len(assets) == 1:
             data["asset"] = assets[0]
             yield data