Fix prom get counter incrementing in external sharded cache

Author: Fizzadar

synapse/replication/tcp/external_sharded_cache.py

@@ -191,7 +191,7 @@ async def mget(self, cache_name: str, keys: Iterable[str]) -> Dict[str, Any]:
 
         logger.debug("Got cache result %s %s: %r", cache_name, keys, combined_results)
 
-        get_counter.labels(cache_name, result is not None).inc()
+        get_counter.labels(cache_name).inc(len(combined_results))
 
         return combined_results