Motivation
As the block cache rolls out we need fleet-level visibility into adoption: how many clusters currently have it enabled. This drives rollout tracking and regression detection.

Approach

• Consume the enabled/disabled stat (from the "report whether the block cache is enabled/disabled" ticket) once it lands in FTDC / the metrics pipeline.
• Build a Grafana panel showing the count of clusters reporting the block cache as enabled over time.

Definition of Done

• A Grafana panel exists showing the number of clusters with the block cache enabled.
• The panel is backed by the enabled stat and refreshes on the standard fleet cadence.

Note: this is an observability/cloud-monitoring task and may need a cloud/observability owner; it is tracked here so the full production effort lives under one epic.