Allow toggling auth for prometheus metrics

Equivalent to jupyterhub/jupyterhub#2224.

Port of jupyter/notebook#5870

Prometheus metrics can potentially leak information about
the user, so they should be kept behind auth by default.
However, for many JupyterHub deployments, they would need
to be scraped by a centralized Prometheus instance that can not
really authenticate separately to each user notebook without
a lot of work. Admins can use this setting to allow unauthenticated
access to the /metrics endpoint.

jupyter_server/base/handlers.py

@@ -842,10 +842,12 @@ def get(self):
 
 class PrometheusMetricsHandler(JupyterHandler):
     """
-    Return prometheus metrics for this Jupyter server
+    Return prometheus metrics for this notebook server
     """
-    @web.authenticated
     def get(self):
+        if self.settings['authenticate_prometheus'] and not self.logged_in:
+            raise web.HTTPError(403)
+
         self.set_header('Content-Type', prometheus_client.CONTENT_TYPE_LATEST)
         self.write(prometheus_client.generate_latest(prometheus_client.REGISTRY))
 

jupyter_server/serverapp.py

@@ -246,6 +246,7 @@ def init_settings(self, jupyter_app, kernel_manager, contents_manager,
             disable_check_xsrf=jupyter_app.disable_check_xsrf,
             allow_remote_access=jupyter_app.allow_remote_access,
             local_hostnames=jupyter_app.local_hostnames,
+            authenticate_prometheus=jupyter_app.authenticate_prometheus,
 
             # managers
             kernel_manager=kernel_manager,
@@ -1199,6 +1200,14 @@ def _update_server_extensions(self, change):
          is not available.
          """))
 
+    authenticate_prometheus = Bool(
+        True,
+        help=""""
+        Require authentication to access prometheus metrics.
+        """,
+        config=True
+    )
+
     def parse_command_line(self, argv=None):
 
         super(ServerApp, self).parse_command_line(argv)