feat(dashboard): add Hostname template variable and update panel legend labels (fixes #630)

[cluster2600]

What

Add a Hostname template variable to the Grafana dashboard and update all panel legend formats to include the hostname alongside the GPU index.

Changes in grafana/dcgm-exporter-dashboard.json:

• New template variable hostname sourced from label_values(DCGM_FI_DEV_GPU_TEMP, Hostname), supporting multi-select and All
• All panel PromQL queries now include Hostname=~"$hostname" as an additional filter
• Legend format updated from GPU {{gpu}} → {{Hostname}}-GPU {{gpu}} for all time-series panels

Why

Closes #630

In multi-node GPU clusters the existing dashboard labels series as GPU 0, GPU 1 … without any host information, making it impossible to tell which physical node a GPU belongs to. When multiple nodes each report a GPU 0, graphs become ambiguous and misleading.

The Hostname label is already exported by dcgm-exporter (controlled by the -n flag). Surfacing it in the dashboard label eliminates the confusion with zero configuration changes on the exporter side.

How

• Added a query-type Grafana template variable that dynamically discovers all hostnames via label_values(DCGM_FI_DEV_GPU_TEMP, Hostname). Supports includeAll and multi so operators can scope the view to a single node or compare across nodes.
• Injected Hostname=~"$hostname" into every panel's PromQL selector so the hostname variable drives real-time filtering.
• Changed legendFormat for the six time-series panels that showed only GPU {{gpu}} to {{Hostname}}-GPU {{gpu}}. Aggregate/stat panels (Avg Temp, Power Total) that had no legend format are left unchanged.

Testing

• JSON validated with python3 -c "import json; json.load(open('grafana/dcgm-exporter-dashboard.json'))"
• Confirmed template variable list order: datasource → hostname → instance → gpu
• Verified all eight panels have the updated PromQL selectors and the six time-series panels have the updated legend format

Checklist

• JSON is valid and imports cleanly into Grafana
• Backward compatible: the Hostname=~".*" default (All) matches all existing metrics unchanged
• No exporter code changes required — Hostname label is already present in default metrics
• Docs not applicable (dashboard is self-documenting in Grafana)