feat(rerank): observe max_chars_per_doc truncation via operation metrics

[chethanuk]

Summary

Follow-up to the max_chars_per_doc cap (#2880). A too-tight cap silently reshapes reranker input; this PR makes it visible in production.

When the cap actually shortens documents inside _rerank_scores, the retriever emits two operation-scoped telemetry counters — rerank.docs_truncated (documents shortened) and rerank.chars_trimmed (characters removed) — which flow through the existing operation-telemetry summary into the Prometheus bridge, mirroring the existing vector.* counters exactly.

How it works

flowchart LR
    A["_rerank_scores (cap > 0)"] --> B["_record_truncation(documents, cap)"]
    B -- "only if docs_truncated &gt; 0" --> C["telemetry.count(<br/>rerank.docs_truncated,<br/>rerank.chars_trimmed)"]
    C --> D["operation telemetry summary<br/>(new rerank section)"]
    D --> E["Prometheus bridge<br/>curated allowlist, emits when value &gt; 0"]
    E --> F["openviking_rerank_docs_truncated_total<br/>openviking_rerank_chars_trimmed_total"]
    F --> G["PromQL: series ABSENT when nothing truncated<br/>coalesce with '... or vector(0)'"]

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Notes

• Metrics only — zero behavior change. Emitted only when something is actually trimmed, so a disabled cap (default 0) or an untriggered cap stays silent (no metric noise). Because the bridge emits only when value > 0, an untruncated run leaves the series absent (not 0) — documented, with the or vector(0) PromQL idiom for alerts.
• Safe when no telemetry is bound: get_current_telemetry() returns a disabled no-op collector whose count() is guarded.
• Distinct from the existing per-call openviking_rerank_* metrics (which use account_id/provider/model_name labels) — no name/registration collision.

Tests / docs

Wired end-to-end with coverage at each hop: retriever counters (emit / silent-when-disabled / silent-when-untriggered / sum-across-docs), counter→summary mapping, and the Prometheus bridge render. Metrics-doc rows added (docs/en|zh/concepts/12-metrics.md). Unit suite green; ruff clean.

Review

Four inline review-bot comments triaged and independently verified. Valid ones fixed (metrics docs corrected to "emitted only on truncation; series absent, not 0" + PromQL note; docstring corrected to name both counters); cap or 0 and a micro-optimization "duplicate scan" declined with rationale (the field is never None; the extra pass over ~20 short abstracts is negligible and the readable split is preferable). No production logic changed.

Stacked on the max_chars_per_doc PR (#2999) — it depends on the char cap. Please review/merge #2999 first; until then this PR's diff includes the v0 commits. Part of #2880.

[chethanuk]

Please review cc: @qin-ctx @ZaynJarvis @zhoujh01 @yufeng201 @chenjw - Please let me know if you require any changes, or if the project is currently accepting contributions