Only trigger prometheus if configured (#231)

* fix prometheus integration

* fix prometheus integration

* Add also lowercase option

As they seem to support that too: https://github.com/prometheus/client_python/blob/d7c9cd88c7f50097cd86869974301df7615bc9c0/prometheus_client/multiprocess.py#L166-L167

* format

---------

Co-authored-by: Stan Triepels <1939656+GDay@users.noreply.github.com>

django_q/cluster.py

@@ -1,4 +1,5 @@
 # Standard
+import os
 import signal
 import socket
 import uuid
@@ -230,8 +231,14 @@ class Sentinel:
                 % {"name": process.name}
             )
         else:
-            if prometheus_multiprocess:
+            # check if prometheus is proper configurated
+            prometheus_path = os.getenv(
+                "PROMETHEUS_MULTIPROC_DIR", os.getenv("prometheus_multiproc_dir")
+            )
+
+            if prometheus_multiprocess and prometheus_path:
                 prometheus_multiprocess.mark_process_dead(process.pid)
+
             self.pool.remove(process)
             self.spawn_worker()
             if process.timer.value == 0: