[Core][Metrics] Remove vllm:prompt_tokens_recomputed metric

[markmc]

In the case of a full local prefix cache hit (prompt length N), we actually only use N-1 tokens. The vllm:prompt_tokens_recomputed was intended to count how many cached tokens we are effectively discarding because of this.

KVCacheManager.get_computed_blocks():
    ...
    # NOTE: When all tokens hit the cache, we must recompute the last token
    # to obtain logits. [...]
    max_cache_hit_length = request.num_tokens - 1

However, even here, we can't assume the last token would have been a cache hit and should be counted as "recomputed". Given this, the metric seems quite misguided, in retrospect.

The metric was added as a side-effect in #33290 in order to make sense of the fact that:

vllm:prompt_tokens_by_source_total{source="external_kv_transfer"}

will include a token that is recomputed. See this comment:

Note: external_kv_transfer reports the actual number of tokens
transferred (e.g., prompt length N), while prompt_tokens_cached_total
reports the adjusted count (e.g., N-1). The last token is both
transferred AND recomputed locally, so there's overlap.

However, it makes more sense for the external_kv_transfer count to reflect only tokens we actually used, not any recomputed tokens. This will be done in ##37460.

I'm not aware of any user demand for this metric, or anyone relying on it now. So it seems safe to remove it, rather than go through a deprecation period.

[markmc]

/cc @ZhanqiuHu

[gemini-code-assist]

Code Review

This pull request removes the tracking and reporting of recomputed tokens from the metrics system. Specifically, it deletes the vllm:prompt_tokens_recomputed metric, simplifies the PromptTokenStats class by removing the recomputation logic and its associated invariants, and updates the relevant tests to reflect these changes. I have no feedback to provide.

[mergify]

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