[Frontend] Add cache salt and tracing to structured decisions (#60786)

Signed-off-by: Taewoong Kim <ktw2172@gmail.com>
Co-authored-by: Chauncey <chaunceyjiang@gmail.com>

docs/serving/online_serving/structured_decisions.md

@@ -135,6 +135,11 @@ the request lists them, a `noul` labels its options `yes` and `no`, and a
 | `questions` | question id to `{type, instructions, criteria}`, asked in this order |
 | `instructions` | optional context placed ahead of the questions |
 | `chat_template_kwargs` | passed to the chat template, for example `{"enable_thinking": false}` |
+| `cache_salt` | optional [prefix-cache salt](../../usage/security.md#cache-salting) applied to every question |
 
 A question with any field other than `type`, `instructions` and `criteria` is
 rejected with a 400.
+
+When [tracing](../../../examples/observability/opentelemetry/README.md) is enabled
+with `--otlp-traces-endpoint`, the HTTP `traceparent` and `tracestate` headers are
+forwarded to every question's generation request.

docs/usage/security.md

@@ -572,7 +572,7 @@ An attacker sharing the same backend can measure differences in Time to First To
 
 vLLM accepts an optional `cache_salt` parameter on requests. The salt is mixed into the hash of the first KV cache block, so only requests carrying the same salt can share cached prefix blocks. See [Automatic Prefix Caching](../design/prefix_caching.md) for the implementation details.
 
-`cache_salt` is accepted by the OpenAI-compatible chat completions, completions, responses, and pooling (embeddings, classification, scoring) endpoints, and by the Anthropic `/v1/messages` endpoint.
+`cache_salt` is accepted by the OpenAI-compatible chat completions, completions, responses, and pooling (embeddings, classification, scoring) endpoints, the Anthropic `/v1/messages` endpoint, and the structured decisions `/v1/systemone` endpoint.
 
 #### Usage with the OpenAI Python client