Add account usage logic (#1567)

---------

Co-authored-by: Yury Gargay <yury.gargay@gmail.com>

management/server/store.go

@@ -1,6 +1,7 @@
 package server
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -41,6 +42,7 @@ type Store interface {
 	// GetStoreEngine should return StoreEngine of the current store implementation.
 	// This is also a method of metrics.DataSource interface.
 	GetStoreEngine() StoreEngine
+	CalculateUsageStats(ctx context.Context, accountID string, start time.Time, end time.Time) (*AccountUsageStats, error)
 }
 
 type StoreEngine string