Add RedisQueueCollector for gathering information about Celery queues

Trying to extract this information from Celery itself is painful and
Celery itself recommends gathering this information from the broker (in
srht case Redis) directly.

srht/metrics.py

@@ -0,0 +1,32 @@
+import time
+from prometheus_client.metrics_core import GaugeMetricFamily
+from redis import Redis, ResponseError
+
+
+class RedisQueueCollector:
+    def __init__(self, broker, name, documentation, queue_name="celery"):
+        self.redis = Redis.from_url(broker)
+        self.queue_name = queue_name
+        self.name = name
+        self.documentation = documentation
+
+    def collect(self):
+        start = time.time()
+        errors = 0
+        try:
+            queue_size = self.redis.llen(self.queue_name)
+        except ResponseError: # Key is not a list
+            queue_size = 0
+            errors += 1
+        duration = time.time() - start
+        yield GaugeMetricFamily(self.name + "_queue_length", self.documentation, value=queue_size)
+        yield GaugeMetricFamily(
+            self.name + "_queue_length_collection_time_seconds",
+            "Time to collect queue metrics",
+            value=duration,
+        )
+        yield GaugeMetricFamily(
+            self.name + "_queue_length_collection_error_count",
+            "Errors encountered while collecting queue metrics",
+            value=errors,
+        )