fix(go): add automatic telemetry metrics for background models

[sahdev77]

Add support for calculating and tracking usage metrics (input/output characters, images, videos, audio) and latency for background model operations. Metrics are now properly calculated when long-running operations complete, not when they start.

Help the reviewer by:

• Modified background_model.go to remove addAutomaticTelemetry() middleware from the Start operation, since metrics should only be calculated when operations complete
• Added CalculateInputOutputUsage() to reuse in background model plugin.
• Updated VEO model implementation to store input request and start time in operation metadata during Start
• Added telemetry calculation in VEO check function that computes both input/output usage metrics and latency when operation completes
• Metadata restoration ensures input metrics and start time are preserved across operation status checks

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually via VEO sample)
• Docs updated (updated docs or a docs bug required)