fix(metrics): Accurately track emitted tokens for speculative decoding

[Ravindranath-Porandla]

Description

This fixes #56101 where the speculative decoding mean_acceptance_length overestimates emitted tokens in cases where the generation is truncated (e.g. by an early EOS token or max limit constraints).

The calculation now explicitly tracks the exact number of emitted tokens from the length of new_token_ids during scheduler updates rather than assuming 1 + accepted_drafts_count. SpecDecodingStats has also been updated to aggregate num_emitted_tokens explicitly.

AI Assistance Statement:
This PR was authored with the assistance of Claude Code.

Test Results

• Ran uv run pytest tests/v1/spec_decode/test_request_acceptance.py -> 11/11 tests pass successfully.
• Code conforms to ruff style guidelines.

Checklist

• Pre-commit hooks run successfully
• Unit tests added/updated for speculative decoding metrics
• Human submitter has reviewed and verified the code and tests

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[seongyun1104]

Reading this against main (9a35c081e8) for the terminal-step question, I think moving the grammar adjustment ahead of the guard in make_spec_decoding_stats changes more than the refactor intends.

On this branch the caller passes the already-adjusted count:

spec_observe_args = (adj_draft_tokens, num_accepted)
...
spec_decoding_stats = self.make_spec_decoding_stats(
    spec_decoding_stats,
    num_draft_tokens=spec_observe_args[0],
    ...
)

and the guard now tests that adjusted value:

def make_spec_decoding_stats(self, spec_decoding_stats, num_draft_tokens, ...):
    if not self.log_stats or not num_draft_tokens:
        return None

spec_decoding_stats is not per-request state. It is initialised once per update_from_output at :1896 and accumulates across every request in the loop before make_stats consumes it at :2283. So a return None here does not skip one request — it discards what every earlier request in the same step already contributed, and the assignment writes the None back.

On main the guard cannot fire on that condition, because it sees the raw length and the subtraction happens after it:

if not self.log_stats or not num_draft_tokens:      # raw len(), >= 1 whenever this branch runs
    return None
...
if num_invalid_spec_tokens:
    num_draft_tokens -= num_invalid_spec_tokens.get(request_id, 0)

The adjusted count does reach zero in serving. update_draft_token_ids_in_output pads rather than drops when the grammar rejects everything, so the scheduled entry stays non-empty while the invalid count equals it:

# scheduler.py:2451-2466
orig_num_spec_tokens = len(placeholder_spec_tokens)
...
    spec_token_ids = metadata.grammar.validate_tokens(spec_token_ids)   # may return []
num_invalid_tokens = orig_num_spec_tokens - len(spec_token_ids)
if num_invalid_tokens:
    spec_token_ids.extend([-1] * num_invalid_tokens)
    num_invalid_spec_tokens[req_id] = num_invalid_tokens

I had claimed the opposite in #56278 and was corrected there by the author, who is right: with structured output, one request whose drafts are all invalidated gives adj_draft_tokens == 0.

The failure is quiet and order-dependent. In a batch mixing structured-output and plain requests, whether a step reports spec-decoding stats at all depends on where the fully-invalidated request sits in the loop, and the Prometheus counters and the log line just under-report. It needs guided decoding to trigger, which is common enough in production that I do not think it stays rare.

The smallest fix that keeps your refactor is to stop conflating "nothing to record for this request" with "return the accumulator": guard on self.log_stats only, and skip observe_draft when the adjusted count is zero while still returning spec_decoding_stats unchanged. Passing the raw length for the guard and the adjusted one for the observation would also work.

Worth noting for whoever reviews the two fixes for #56101 together: #56137 keeps the subtraction inside the callee, so it does not have this path.

[CarrotSwordsman]

Cross-referencing for awareness: this PR and #56137 both target #56101, but the make_spec_decoding_stats surface you are both modifying is also tracked from a different angle in #34734 and PR #34757 — neither of which is referenced from either PR body.

Two notes from that thread that seem directly relevant:

1. The guard-ordering hazard @seongyun1104 flagged above is real and I can confirm it against main: spec_decoding_stats is initialized once per update_from_output and the call site assigns the return value back inside the per-request loop, so a mid-loop return None after the grammar adjustment discards every request already folded in for that step — silently and order-dependently. The original residual finding (comment) was that make_spec_decoding_stats guards num_draft_tokens before subtracting num_invalid_spec_tokens, inflating the drafts counter on the async-scheduling and structured-output paths; the fix direction we recommended there is guard-on-raw-count, subtract afterwards — which is what [V1] Fix speculative decoding mean acceptance length for terminal steps #56137 does.

2. If [Bug][Spec Decode] Fix AR metrics when drafting is skipped #34757 (drafter-skip suppression) eventually rebases and lands, it will meet whatever guard shape survives here at the same call site, so aligning the two now avoids a second round of conflicts.