Examples: add oximeter health panels. (#56)

Add dashboard panels showing oximeter producer cardinality, dropped
samples, and queue health, recently added in r21.

The queue depth heatmap may be a little much, but I like it! Changes are
live at https://grafana.monitoring.oxeng.dev/d/oxide-receiver-health.

<img width="1363" height="662" alt="Screenshot 2026-07-13 at 11 59
24 AM"
src="https://github.com/user-attachments/assets/b511f513-3165-44db-a578-01066236253d"
/>

I'm just going to merge this, but flagging for @lgfa29 and @bnaecker in
case relevant to your interests.

Author: jmcarp