Added metrics for fission mqtrigger and optimizations in trigger subscriptions (#2399)

- Use mqtrigger watch instead of polling mqtrigger every 5 seconds
- Added metrics to monitor no of subscriptions, and no of messages per subscription
- Add standard go metrics exported by prometheus
- Enable prometheus discovery for mqtrigger pod
- Optimized mqtrigger manager cache
- Add unit tests for mqtrigger cache

Co-authored-by: Sanket Sudake <sanketsudake@gmail.com>

charts/fission-all/templates/mqt-fission-kafka/deployment.yaml

@@ -18,6 +18,10 @@ spec:
       labels:
         svc: mqtrigger
         messagequeue: kafka
+      annotations:
+        prometheus.io/scrape: "true"
+        prometheus.io/path: "/metrics"
+        prometheus.io/port: "8080"
     spec:
       containers:
       - name: mqtrigger