Add Support for Cache and S3 related metrics in Prometheus endpoint (#8591)

This PR adds support below metrics

- Cache Hit Count
- Cache Miss Count
- Data served from Cache (in Bytes)
- Bytes received from AWS S3
- Bytes sent to AWS S3
- Number of requests sent to AWS S3

Fixes #8549

cmd/gateway-metrics.go

@@ -0,0 +1,79 @@
+/*
+ * MinIO Cloud Storage, (C) 2019 MinIO, Inc.
+ *
+ * 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 cmd
+
+import (
+	"sync"
+
+	"go.uber.org/atomic"
+)
+
+// Metrics - represents bytes served from backend
+// only implemented for S3 Gateway
+type Metrics struct {
+	BytesReceived atomic.Uint64
+	BytesSent     atomic.Uint64
+	RequestStats  map[string]int
+	sync.RWMutex
+}
+
+// IncBytesReceived - Increase total bytes received from gateway backend
+func (s *Metrics) IncBytesReceived(n int64) {
+	s.BytesReceived.Add(uint64(n))
+}
+
+// GetBytesReceived - Get total bytes received from gateway backend
+func (s *Metrics) GetBytesReceived() uint64 {
+	return s.BytesReceived.Load()
+}
+
+// IncBytesSent - Increase total bytes sent to gateway backend
+func (s *Metrics) IncBytesSent(n int64) {
+	s.BytesSent.Add(uint64(n))
+}
+
+// GetBytesSent - Get total bytes received from gateway backend
+func (s *Metrics) GetBytesSent() uint64 {
+	return s.BytesSent.Load()
+}
+
+// IncRequests - Increase request sent to gateway backend by 1
+func (s *Metrics) IncRequests(method string) {
+	s.Lock()
+	defer s.Unlock()
+	if s == nil {
+		return
+	}
+	if s.RequestStats == nil {
+		s.RequestStats = make(map[string]int)
+	}
+	if _, ok := s.RequestStats[method]; ok {
+		s.RequestStats[method]++
+		return
+	}
+	s.RequestStats[method] = 1
+}
+
+// GetRequests - Get total number of requests sent to gateway backend
+func (s *Metrics) GetRequests() map[string]int {
+	return s.RequestStats
+}
+
+// NewMetrics - Prepare new Metrics structure
+func NewMetrics() *Metrics {
+	return &Metrics{}
+}