docs: rewrite metrics.md for current EPP subsystems

[chethanuk]

User description

What type of PR is this?

/kind documentation

What this PR does / why we need it:

docs/metrics.md stated that all metrics live under the
llm_d_inference_scheduler subsystem, which is now deprecated after the
router rename and metrics migration. This rewrites the guide against the
current code:

• Subsystems and naming: llm_d_epp_ is the canonical EPP-wide prefix;
  llm_d_router_epp_ is used only by the embedded llm-d-kv-cache metrics.
  The legacy prefixes (llm_d_inference_scheduler_, inference_objective_,
  inference_pool_, inference_extension_, kvcache_) are deprecated but
  still dual-emitted, with a replacement table.
• Scrape topology for operators: the EPP :9090/metrics endpoint
  (controller-runtime registry), the model-server data-layer scrape, and the
  embedded kv-cache metrics that register on the same registry when
  indexerConfig.kvBlockIndexConfig.enableMetrics: true (default false) — no
  separate kv-cache endpoint.
• Expanded catalog beyond disaggregation: request/latency, pool, scheduler,
  plugin, data-layer errors, prefix indexer, multimodal, program-aware
  fairness, predicted latency, plus the existing flow-control and ext_proc
  sections, each with types and labels.
• Cross-links to related observability work (Align on name/label for plugin scoped metrics llm-d/llm-d-router#1243, [Deprecation]: Deprecate old metrics llm-d/llm-d-router#1070, [deprecation] Remove lagacy data and metrics collection llm-d/llm-d-router#962, llm-d router (particularly EPP) operations guide llm-d/llm-d-router#1291, [Testing] Resolve EPP Metrics Verification Flakiness in E2E Test Suites llm-d/llm-d-router#1192).

Notes for reviewers on the issue's wording vs the code: the canonical EPP-wide
prefix is llm_d_epp_*, not llm_d_router_epp_* (which is kv-cache-only); and
pd_decision_total / disagg_decision_total are both current llm_d_epp_*
metrics — the actual deprecation is llm_d_inference_scheduler_* -> llm_d_epp_* per metric. Every documented metric name was cross-checked against
its registration in pkg/. The e2e kv-cache metrics assertion (issue item 4)
is left as the noted follow-up.

Which issue(s) this PR fixes:

Fixes llm-d#1394

Release note:

NONE

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CodeAnt-AI Description

Update the metrics guide for the current EPP metrics and scrape setup

What Changed

• Replaces the outdated single-prefix view with the current EPP metric names and shows which legacy names are still emitted during migration
• Adds the current scrape layout: the EPP /metrics endpoint, model-server data-layer metrics, and embedded kv-cache metrics on the shared registry
• Expands the catalog to cover request, pool, scheduler, plugin, data-layer, flow control, ext_proc, prefix cache, multimodal, fairness, predicted latency, and kv-cache metrics
• Clarifies the deprecated pd_decision_total behavior and points readers to the current disaggregation metric
• Adds links to related observability work

Impact

✅ Easier metrics migration
✅ Clearer operator scrape setup
✅ Fewer mismatches between docs and live metrics

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Status	Commit	Started (UTC)	Finished (UTC)
✅ Reviewed your PR	7ba7025	Jul 19, 2026 · 11:59	12:00

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Code Review

This pull request updates and expands the documentation in docs/metrics.md to comprehensively detail the Prometheus metrics exposed by the llm-d-router Endpoint Picker (EPP). It introduces structured sections explaining subsystems and naming prefixes, scrape topologies, deprecated series, and a detailed metrics catalog categorized by functional areas such as flow control, disaggregation, and the KV-cache index. No review comments were provided, so there is no additional feedback to address.

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