add comment about why unreg

synapse/util/caches/__init__.py

@@ -41,6 +41,8 @@ response_cache_total = Gauge("synapse_util_caches_response_cache:total", "", ["n
 def register_cache(cache_type, cache_name, cache):
 
     # Check if the metric is already registered. Unregister it, if so.
+    # This usually happens during tests, as at runtime these caches are
+    # effectively singletons.
     metric_name = "cache_%s_%s" % (cache_type, cache_name)
     if metric_name in collectors_by_name.keys():
         REGISTRY.unregister(collectors_by_name[metric_name])