What is the bracket '[]' Datasource in Query Metrics?

We’ve recently upgraded to Druid 0.22.1, and have noticed an odd Datasource being reported in query metrics. The Datasource is named [] as though it is an empty list. This is not a real Datasource that exists in Druid, and so we are wondering where this comes from.

We are currently using the statsd-emitter on version 0.22.1. We did not notice this in Druid 0.21, and it is not present in Druid 0.19 which we still have deployed. It gets queried infrequently. We’ve noticed that query times to it increase dramatically (up to 1-5 mins per query) when the Brokers are running at high CPU (80% or more).

Could someone explain what this Datasource is? Is this a statsd-emitter artifact, or something new in Druid 0.22? Should we be paying attention to this? Should we be concerned when queries to it are taking a long time?

Thanks for any help!