Run Prometheus on a different port, optionally. (#3274)

synapse/metrics/__init__.py

@@ -39,7 +39,8 @@ HAVE_PROC_SELF_STAT = os.path.exists("/proc/self/stat")
 
 class RegistryProxy(object):
 
-    def collect(self):
+    @staticmethod
+    def collect():
         for metric in REGISTRY.collect():
             if not metric.name.startswith("__"):
                 yield metric

synapse/metrics/resource.py

@@ -13,4 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from prometheus_client.twisted import MetricsResource
+
 METRICS_PREFIX = "/_synapse/metrics"
+
+__all__ = ["MetricsResource", "METRICS_PREFIX"]