We don't currently have any telemetry for encryption at rest. Before the release of 19.1, we should add a feature counter to our storage engine to indicate whether encryption is enabled on each store or not. It doesn't look like there are any existing feature counters in this part of the codebase. This is probably just a check that we should run on each store at node startup; we can aggregate the results by cluster on the server side after collecting the individual encrypted/not-encrypted counts.