do-agent: add DI metrics whitelist

[sk09-dev]

This PR adds and syncs Dedicated Inference (DI) metrics support in do-agent.

Changes included:

• Added DI metrics whitelist for GPU, objective counters, and vLLM metrics
• Added --di-metrics-path flag to enable DI metrics scraping
• Added aggregation rules to drop high-cardinality labels while preserving histogram buckets (le)
• Consolidated earlier work into a single clean PR (earlier PRs closed)

Validation:

• Verified DI vLLM *_bucket metrics are scraped correctly
• Confirmed histogram labels (le) are preserved
• Confirmed high-cardinality labels are dropped via aggregation as expected