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>

pkg/mqtrigger/metrics.go

@@ -0,0 +1,53 @@
+/*
+Copyright 2022 The Fission Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package mqtrigger
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	metricsAddr       = ":8080"
+	labels            = []string{"trigger_name", "trigger_namespace"}
+	subscriptionCount = promauto.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Name: "fission_mqt_subscriptions",
+			Help: "Total number of subscriptions to mq currently",
+		},
+		[]string{},
+	)
+	messageCount = promauto.NewCounterVec(
+		prometheus.CounterOpts{
+			Name: "fission_mqt_messages_processed_total",
+			Help: "Total number of messages processed",
+		},
+		labels,
+	)
+)
+
+func IncreaseSubscriptionCount() {
+	subscriptionCount.WithLabelValues().Inc()
+}
+
+func DecreaseSubscriptionCount() {
+	subscriptionCount.WithLabelValues().Dec()
+}
+
+func IncreaseMessageCount(trigname, trignamespace string) {
+	messageCount.WithLabelValues(trigname, trignamespace).Inc()
+}