Add a configurable prometheus namespace for all metrics

Fixes #1918

config.py

@@ -369,6 +369,9 @@ class DefaultConfig(object):
   # URL that specifies the location of the prometheus stats aggregator.
   PROMETHEUS_AGGREGATOR_URL = 'http://localhost:9092'
 
+  # Namespace prefix for all prometheus metrics.
+  PROMETHEUS_NAMESPACE = 'quay'
+
   # Reverse DNS prefixes that are reserved for internal use on labels and should not be allowable
   # to be set via the API.
   DEFAULT_LABEL_KEY_RESERVED_PREFIXES = ['com.docker.', 'io.docker.', 'org.dockerproject.',