We are a startup, can someone please suggest the best way/practices to monitor consumer lag?

You can setup a grafana dashboard with prometheus to monitor your consumer lag

We want subscription-level monitoring/alert, is there an outbox support?

I believe the default ones in the pulsar-helm-chart do have subscription level metrics

would be a great help if you could share any references?

Sure, you can refer to the documentation here: https://github.com/apache/pulsar-helm-chart?tab=readme-ov-file#grafana-dashboards Looks like the helmchart installs it by default if you are not disabling it: https://github.com/apache/pulsar-helm-chart/blob/master/charts/pulsar/values.yaml#L1183