grafana: include hostname in GPU legends

[harsh543]

Summary

• include the exporter hostname label in GPU panel legends
• update GPU time-series legends from GPU {{gpu}} to {{hostname}} - GPU {{gpu}}

Why

In multi-node clusters, every node can have gpu="0", gpu="1", etc. Showing only the GPU index makes dashboard series ambiguous. Including hostname lets operators distinguish the node and GPU directly in the legend.

Fixes #630.

Validation

• python3 -m json.tool grafana/dcgm-exporter-dashboard.json >/dev/null
• git diff --check

[harsh543]

Hey @nccurry — you merged #655 which renamed Hostname → hostname, so you're the right person for this one.

What this fixes: The shipped Grafana dashboard uses {{Hostname}} (capital H) in GPU panel legend templates. After the label rename in #655, that variable resolves to nothing — all GPU panels show blank legends in multi-node clusters.

The fix: Updates the legend template strings in the dashboard JSON from {{Hostname}} → {{hostname}} to match the current label casing.

Closes #630. No metrics or Go changes — dashboard JSON only.