Updated Prometheus metrics (#11141)

* Add metrics for nodes online and offline
* Add cluster capacity metrics
* Introduce v2 metrics

cmd/object-api-interface.go

@@ -72,6 +72,13 @@ const (
 	writeLock
 )
 
+// BackendMetrics - represents bytes served from backend
+type BackendMetrics struct {
+	bytesReceived uint64
+	bytesSent     uint64
+	requestStats  RequestStats
+}
+
 // ObjectLayer implements primitives for object API layer.
 type ObjectLayer interface {
 	SetDriveCount() int // Only implemented by erasure layer
@@ -143,7 +150,7 @@ type ObjectLayer interface {
 	IsCompressionSupported() bool
 
 	// Backend related metrics
-	GetMetrics(ctx context.Context) (*Metrics, error)
+	GetMetrics(ctx context.Context) (*BackendMetrics, error)
 
 	// Returns health of the backend
 	Health(ctx context.Context, opts HealthOptions) HealthResult