feat: add semantic conventions for gce instance labels

[paulojmdias]

Relates to https://github.com/open-telemetry/opentelemetry-collector-contrib/pull/41919

Changes

This change introduces a new templated attribute for Google Compute Engine instance labels. The attribute prefix gcp.gce.instance.labels. follows the existing GCP/GCE naming scheme and aligns with current semantic conventions for similar resources (e.g., container.label).

Merge requirement checklist

• [X] CONTRIBUTING.md guidelines followed.
• [X] Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• [X] Links to the prototypes or existing instrumentations (when adding or changing conventions)