Add targetClusterName label for Prometheus XDCR Metrics

All of the XDCR metrics contain 4 common labels: 

• targetClusterUUID
• sourceBucketName
• targetBucketName
• pipelineType

xdcr_changes_left_total {targetClusterUUID="a276ef776134d7f0efc5875e881bff55", sourceBucketName="travel-sample", targetBucketName="demo", pipelineType="Main"} 0 

When visualizing XDCR metrics in Grafana or alerting based on certain XDCR metrics, labels are used to filter and/or add context for each metric.  No customer will ever know what the Cluster UUID is, especially if they have multiple remote clusters added and will be very difficult for them to make sense of given metrics.  

A new label of targetClusterName should be added that is the value of what was populated when the user created the remote replication.