document the "origin" label in queue populator metrics

Issue: BB-527

Author: Kerkesni

lib/queuePopulator/QueuePopulator.js

@@ -23,6 +23,10 @@ const {
     startCircuitBreakerMetricsExport,
 } = require('../CircuitBreaker');
 
+// origin is the name of the workflow that triggered the processing
+// of the log entries. The label can include multiple values when
+// the queue populator is shared between multiple workflows (e.g.
+// replication, lifecycle, notification).
 const metricLabels = ['origin', 'logName', 'logId'];
 
 /**