The problem
Horizon tells you how many jobs are waiting. It does not tell Prometheus, so a platform team either scrapes Redis itself or finds out about a queue backlog when a customer does.
What it is
QueueLens is a small exporter that reads Horizon's Redis structures and publishes queue depth, oldest-job age, throughput and failure counts as Prometheus metrics. A bundled Grafana dashboard turns those into a queue-depth SLO.
Design rule
Every metric names its queue and its connection. An aggregate that hides which queue is backed up is a dashboard that looks healthy while one worker starves.
Outcome
Four internal teams adopted it in the first quarter, and it now ships with the default platform image, so a new service gets queue visibility without adding an exporter.