Add Loki clusterLabel to Loki logs shipped with promtail

Feature Request

Why

Right now, all of the Loki logs that are shipped to Loki from promtail don't contain the "cluster" tag making selfMonitoring dashboards useless.

Proposed Solution

Add a rewrite in promtail to add the "cluster=" label to logs generated by Loki pods before being shipped to Loki storage.