Add metrics to runner

[metascroy]

Adds timing instrumentation to the batched runner, which previously reported nothing. metrics.h defines two tiers: GenerationMetrics, published on GenerationHandle beside finish_reason(), covering the submit→first-step→first-token→end timeline, token counts, and an inter-token latency summary; and EngineMetrics, owned by the runner and read after shutdown(), covering step counts and latency, decode/prefill sequences and tokens, admitted-versus-ready decode sequences, and TTFT rollup.

[pytorch-bot]

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❌ 1 New Failure, 1 Cancelled Job, 1 Unrelated Failure

As of commit 0d4e638 with merge base 0447246 ():

NEW FAILURE - The following job has failed:

• Lint / lintrunner (gh)
  >>> Lint for extension/llm/batching/runner.cpp:

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-editable / linux / linux-job (gh)

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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