admin: allow disabling display of metrics

This is useful in situations where the admins of the Snikket
server (i.e. those who care for the docker containers) are not the
same people as the people who are admins of the Snikket service
(i.e. those who care for the users).

snikket_web/__init__.py

@@ -160,6 +160,7 @@ class AppConfig:
     # Future versions may change this default, and the standard deployment
     # tools may also very well override it.
     max_avatar_size = environ.var(1024*1024, converter=int)
+    show_metrics = environ.bool_var(True)
 
 
 _UPPER_CASE = "".join(map(chr, range(ord("A"), ord("Z")+1)))
@@ -191,6 +192,7 @@ def create_app() -> quart.Quart:
     app.config["AVATAR_CACHE_TTL"] = config.avatar_cache_ttl
     app.config["APPLE_STORE_URL"] = config.apple_store_url
     app.config["MAX_AVATAR_SIZE"] = config.max_avatar_size
+    app.config["SHOW_METRICS"] = config.show_metrics
 
     app.context_processor(proc)
     app.register_error_handler(