Add validate to metrics backends

Author: dcramer

src/sentry/metrics/base.py

@@ -26,3 +26,6 @@ def incr(self, key, instance=None, tags=None, amount=1, sample_rate=1):
 
     def timing(self, key, value, instance=None, tags=None, sample_rate=1):
         raise NotImplementedError
+
+    def validate(self):
+        pass

src/sentry/utils/runner.py

@@ -353,6 +353,7 @@ def validate_backends():
         app.buffer,
         app.digests,
         app.nodestore,
+        app.metrics,
         app.quotas,
         app.ratelimiter,
         app.search,