bug-1847235: move markus usage to hook function

We don't want the gunicorn manager process to import Tecken webapp
stuff, so this moves importing markus and creating a markus client to
the point where it's used.

tecken/gunicornhooks.py

@@ -7,14 +7,10 @@
 
 See https://docs.gunicorn.org/en/stable/settings.html#server-hooks
 
-Note: Don't import anything that involves Django machinery here.
+Note: This file is imported by the Gunicorn manager, so don't import anything that
+involves Tecken webapp machinery here.
 """
 
-import markus
-
-
-metrics = markus.get_metrics("tecken")
-
 
 def worker_abort(worker):
     """Emit metric when a Gunicorn worker is terminated from timeout
@@ -25,4 +21,7 @@ def worker_abort(worker):
        markus and logging configuration of the Django webapp.
 
     """
+    import markus
+
+    metrics = markus.get_metrics("tecken")
     metrics.incr("gunicorn_worker_abort")