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/disk-cache-stats.go

@@ -0,0 +1,64 @@
+/*
+ * 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 (
+	"go.uber.org/atomic"
+)
+
+// CacheStats - represents bytes served from cache,
+// cache hits and cache misses.
+type CacheStats struct {
+	BytesServed atomic.Uint64
+	Hits        atomic.Uint64
+	Misses      atomic.Uint64
+}
+
+// Increase total bytes served from cache
+func (s *CacheStats) incBytesServed(n int64) {
+	s.BytesServed.Add(uint64(n))
+}
+
+// Increase cache hit by 1
+func (s *CacheStats) incHit() {
+	s.Hits.Add(uint64(1))
+}
+
+// Increase cache miss by 1
+func (s *CacheStats) incMiss() {
+	s.Misses.Add(uint64(1))
+}
+
+// Get total bytes served
+func (s *CacheStats) getBytesServed() uint64 {
+	return s.BytesServed.Load()
+}
+
+// Get total cache hits
+func (s *CacheStats) getHits() uint64 {
+	return s.Hits.Load()
+}
+
+// Get total cache misses
+func (s *CacheStats) getMisses() uint64 {
+	return s.Misses.Load()
+}
+
+// Prepare new CacheStats structure
+func newCacheStats() *CacheStats {
+	return &CacheStats{}
+}