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/api-headers.go

@@ -94,7 +94,7 @@ func setObjectHeaders(w http.ResponseWriter, objInfo ObjectInfo, rs *HTTPRangeSp
 
 	// Set all other user defined metadata.
 	for k, v := range objInfo.UserDefined {
-		if hasPrefix(k, ReservedMetadataPrefix) {
+		if HasPrefix(k, ReservedMetadataPrefix) {
 			// Do not need to send any internal metadata
 			// values to client.
 			continue