app: allow disabling metrics by environment variable

app/__init__.py

@@ -1,3 +1,4 @@
+import os
 from typing import Iterator
 
 from flask import Flask, redirect, url_for
@@ -100,7 +101,7 @@ class AutomationCollector(Collector):
             yield c
 
 
-if not_migrating():
+if not_migrating() and 'DISABLE_METRICS' not in os.environ:
     REGISTRY.register(DefinedProxiesCollector())
     REGISTRY.register(BlockedProxiesCollector())
     REGISTRY.register(AutomationCollector())