Show how much storage space is used by shared files

Requires at least https://hg.prosody.im/prosody-modules/rev/883ad8b0a7c0

snikket_web/admin.py

@@ -682,6 +682,11 @@ async def system() -> typing.Union[str, quart.Response]:
         except KeyError:
             pass
 
+        try:
+            metrics["prosody_uploads"] = prosody_metrics["uploads"]
+        except KeyError:
+            pass
+
         for k in list(metrics.keys()):
             if metrics[k] is None:
                 # so that defaulting in jinja works