observability: rewrite /metrics admin command on Prometheus generate_latest (#2058 slice 1/2)

[yastman]

This pull request was created by @kiro-agent on behalf of @yastman 👻

Comment with /kiro fix to address specific feedback or /kiro all to address everything.
_{Learn about Kiro autonomous agent}

---

Parent: #1648 (cleanup) → #2058.

#2058 asks to remove the rolling-window PipelineMetrics surface after Prometheus counters/histograms become authoritative. The full removal requires migrating ~7 PipelineMetrics.get() call-sites (cache.py, rerank.py, retrieve.py, generate_response.py) and ~5 unit-test modules that mock PipelineMetrics. That's a large multi-file change.

This PR delivers slice 1/2: rewrite the user-facing admin /metrics Telegram command on the SDK-native generate_latest() exposition format. It isolates the user-visible change from the internal class removal.

Before

async def cmd_metrics(bot: PropertyBot, message: Message) -> None:
    """Handle /metrics command - show pipeline p50/p95 timing stats."""
    metrics = PipelineMetrics.get()
    text = f"```\n{metrics.format_text()}\n```"
    await message.answer(text, parse_mode="Markdown")

After

async def cmd_metrics(bot: PropertyBot, message: Message) -> None:
    """Handle /metrics command — emit current Prometheus metrics text format."""
    from prometheus_client import REGISTRY, generate_latest

    payload = generate_latest(REGISTRY).decode("utf-8")
    if len(payload) > 3500:
        payload = payload[:3500] + "\n# ...truncated"
    text = f"```\n{payload}\n```"
    await message.answer(text, parse_mode="Markdown")

User-facing admin behaviour preserved (Markdown code-block, single message). The 3500-char truncation guards Telegram's 4096-char message limit when the registry is large in production.

Tests updated

• tests/unit/handlers/test_command_handlers.py::TestCmdMetrics — asserts on generate_latest() output (Prometheus exposition format markers + a sentinel Counter).
• tests/unit/test_bot_handlers.py::test_cmd_metrics — same pattern.

New contract test

tests/contract/test_admin_metrics_uses_prometheus_contract.py with two invariants:

1. cmd_metrics() body calls generate_latest(...).
2. cmd_metrics() body does not call format_text() (modulo the docstring, which legitimately names the deprecated symbol it replaces).

Slice 2/2 — class removal — deferred

The follow-up that removes the PipelineMetrics class itself (and migrates the remaining 7 call-sites) should land after #2056 merges the record_counter_metric → record_pipeline_event migration. I'll leave a comment on #2058 reflecting that.

Verification

uv run pytest tests/contract/test_admin_metrics_uses_prometheus_contract.py
              tests/unit/handlers/test_command_handlers.py
              tests/unit/test_bot_handlers.py -q
  207 passed

uvx ruff check telegram_bot/handlers/command_handlers.py \
               tests/contract/test_admin_metrics_uses_prometheus_contract.py
  All checks passed!

Refs #2058 (slice 1/2 — class removal in follow-up).