[zhanghaohit]

What

• listener: the per-request details line (metrics/details/<date>.jsonl, what the exporter tails) gets actual_cached_tokens, taken from resp_meta.actual_usage.prompt_tokens_details.cached_tokens.
  • It is written only when that value is a number. A response without actual_usage, or with cached_tokens: null, has no field, so "not reported" and "reported 0" stay apart.
  • The DuckDB details schema gains the column. Older jsonl / parquet files read it as NULL.
• exporter: new counter bodylog_actual_cached_tokens_total, with the same labels as bodylog_cached_tokens_total (service, route, backend, model). Only lines that carry the field feed it.

Why

bodylog_cached_tokens_total counts usage.prompt_tokens_details.cached_tokens, which is what clients and billing see. Some sglang builds also return actual_usage with the engine's own count, and it can differ:

"usage":        {"prompt_tokens": 4096, "prompt_tokens_details": {"cached_tokens": 2048, "cache_write_tokens": 1792}},
"actual_usage": {"prompt_tokens": 4096, "prompt_tokens_details": {"cached_tokens": 3072}}

Since #30 the bodylog keeps actual_usage in resp_meta, but the number reached no metric. With both counters, a dashboard can show the reported and the engine-side hit rate side by side.

Tests

• listener TestDetailActualCachedTokens covers:
  • reported, reported 0, null, no prompt_tokens_details, and absent;
  • cached_tokens stays at usage's figure in every case;
  • the field appears on the JSON line exactly when reported.
• exporter TestActualCachedTokensCounter: three lines with 3072 / 0 / no field. bodylog_cached_tokens_total = 3 × 2048 and bodylog_actual_cached_tokens_total = 3072. Fails on the previous metrics.go.
• go test ./... passes in both modules.