observability: panel-by-panel review of the Grafana dashboard for multi-server/multi-cluster setups

[gabriele-wolfox]

Using the multi-server/multi-cluster observability sample (operator/config/samples/opentelemetry-multi/), review the Klio Grafana dashboard panel by panel.
Until now it's only been validated against a single server/cluster, where nothing is ambiguous.

For each panel, check:

• Aggregation: sum/max/avg/rate/increase don't silently fold multiple clusters or servers into one misleading number.
• Legends: every series clearly identifies cluster, server, tier, stage, path, percentile, etc. No bare, ambiguous, or duplicate labels.
• Usefulness: the metric answers a real operational question. Rework or remove panels that don't; add missing ones where
  attributable.
• Visualization: right chart type/unit, stays readable (e.g. time series over raw stat when history matters).

Fix any broken or misleading panel. Regenerate dashboard JSON with task grafana:gen after changes.