Add basic metrics export

marietje/metrics/admin.py

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+from django.contrib import admin
+
+# Register your models here.

marietje/metrics/apps.py

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+from django.apps import AppConfig
+
+
+class MetricsConfig(AppConfig):
+    name = 'metrics'

marietje/metrics/models.py

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+from django.db import models
+
+# Create your models here.

marietje/metrics/tests.py

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+from django.test import TestCase
+
+# Create your tests here.

marietje/metrics/views.py

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+from prometheus_client import generate_latest, Gauge
+
+from django.db.models import Q
+from django.http import HttpResponse
+
+from queues.models import Queue, PlaylistSong
+
+# Export queue length
+g = Gauge('marietje_queue_length', 'Marietje queue length', ['name'])
+for queue in Queue.objects.all():
+    def _get_queue_length():
+         return PlaylistSong.objects.filter(Q(playlist=queue.playlist_id),
+                                           Q(state=0) | Q(state=1)).count()
+    g.labels(name=queue).set_function(_get_queue_length)
+
+
+def metrics(request):
+    return HttpResponse(generate_latest(), content_type="text/plain")