Don't log GC 0s at INFO (#5557)

changelog.d/5557.misc

@@ -0,0 +1 @@
+Logging when running GC collection on generation 0 is now at the DEBUG level, not INFO.

synapse/metrics/__init__.py

@@ -437,7 +437,10 @@ def runUntilCurrentTimer(func):
         counts = gc.get_count()
         for i in (2, 1, 0):
             if threshold[i] < counts[i]:
-                logger.info("Collecting gc %d", i)
+                if i == 0:
+                    logger.debug("Collecting gc %d", i)
+                else:
+                    logger.info("Collecting gc %d", i)
 
                 start = time.time()
                 unreachable = gc.collect(i)