fix(metrics): Expose dgraph_num_backups_failed_total metric view. (#7900

)

We've been tracking this metric but didn't actually expose it in the metrics
view, so Prometheus metrics in /metrics never showed it.

This change updates sets the view properly so it shows up in /metrics.

(cherry picked from commit 59fd122)

x/metrics.go

@@ -222,6 +222,13 @@ var (
 			Aggregation: view.Count(),
 			TagKeys:     nil,
 		},
+		{
+			Name:        NumBackupsFailed.Name(),
+			Measure:     NumBackupsFailed,
+			Description: NumBackupsFailed.Description(),
+			Aggregation: view.Count(),
+			TagKeys:     nil,
+		},
 		{
 			Name:        TxnCommits.Name(),
 			Measure:     TxnCommits,