Add pod label enrichment support to DCGM Exporter

[karthikvetrivel]

Addresses #2009.

Description

Add support for pod label enrichment in DCGM Exporter metrics. When enabled, GPU metrics will include labels from the pods using those GPUs.

This feature adds:

• New dcgmExporter.podMetrics.enablePodLabels configuration option
• ClusterRole and ClusterRoleBinding for cluster-wide pod read access (conditionally created)
• Automatic ServiceAccount token mounting when enabled
• DCGM_EXPORTER_KUBERNETES_ENABLE_POD_LABELS environment variable injection

The feature is opt-in (disabled by default) since it grants cluster-wide pod read permissions.

Usage

dcgmExporter:
  podMetrics:
    enablePodLabels: true

In draft, needs to be tested.