Fix Django/Datadog integration with Spinach

ddtrace used to be a Django app, which Spinach could look for to
know if it should use the Datadog integration.

Since ddtrace now recommends patching Django, Spinach has to adapt
to this new method to know if it should use the Datadog
integration.

spinach/contrib/datadog.py

@@ -7,8 +7,6 @@ def register_datadog(tracer=None, namespace: Optional[str]=None,
                      service: str='spinach'):
     """Register the Datadog integration.
 
-    Exceptions making jobs fail are sent to Sentry.
-
     :param tracer: optionally use a custom ddtrace Tracer instead of the global
            one.
     :param namespace: optionally only register the Datadog integration for a
@@ -42,3 +40,24 @@ def job_failed(namespace, job, **kwargs):
     def job_schedule_retry(namespace, job, **kwargs):
         root_span = tracer.current_root_span()
         root_span.set_traceback()
+
+
+def register_datadog_if_module_patched(module: str, *args, **kwargs) -> bool:
+    """Register the datadog integration if ddtrace is already used.
+
+    This can be used to enable datadog for Spinach only if datadog
+    is enabled for Django.
+
+    :param module: Name of the module that must already be patched
+    :return: boolean telling if the integration was registered
+    """
+    try:
+        from ddtrace.monkey import get_patched_modules
+    except ImportError:
+        return False
+
+    if module not in get_patched_modules():
+        return False
+
+    register_datadog(*args, **kwargs)
+    return True

spinach/contrib/spinachd/management/commands/spinach.py

@@ -1,4 +1,3 @@
-from django.apps import apps
 from django.core.management.base import BaseCommand
 try:
     from raven.contrib.django.models import client as raven_client
@@ -7,6 +6,7 @@
 
 from spinach.const import DEFAULT_QUEUE, DEFAULT_WORKER_NUMBER
 from spinach.contrib.sentry import register_sentry
+from spinach.contrib.datadog import register_datadog_if_module_patched
 
 from ...apps import spin
 
@@ -41,9 +41,12 @@ def handle(self, *args, **options):
         if raven_client is not None:
             register_sentry(raven_client, spin.namespace)
 
-        if apps.is_installed('ddtrace.contrib.django'):
-            from spinach.contrib.datadog import register_datadog
-            register_datadog(namespace=spin.namespace)
+        # Use the Datadog integration if Datadog is already used
+        # to trace Django.
+        register_datadog_if_module_patched(
+            'django',
+            namespance=spin.namespace
+        )
 
         spin.start_workers(
             number=options['threads'],