List TRACEPARENT and TRACESTATE in the runtime variable reference

Author: SCcagg5

writing-workflows/runtime-variables.md

@@ -73,6 +73,8 @@ Values are refreshed for each step, so `DAG_RUN_STEP_NAME`, `DAG_RUN_STEP_STDOUT
 | `DAG_PUSHBACK` | Steps re-executed after an approval or human-task push-back only | JSON string containing the current push-back iteration, latest inputs, authenticated actor, server timestamp, and chronological history. Not set on the initial execution. | `{"iteration":2,"by":"reviewer","at":"2026-04-26T06:18:43Z","inputs":{"FEEDBACK":"Tighten summary"},"history":[...]}` |
 | `DAG_PUSHBACK_ITERATION` | Steps re-executed after an approval or human-task push-back only | Current push-back iteration as a plain integer string. Not set on the initial execution. | `2` |
 | `DAG_PUSHBACK_PREVIOUS_STDOUT_FILE` | Rewound steps that had stdout before reset | Absolute path to the previous stdout log for the current step. Dagu passes the path instead of inlining stdout because logs can be large. | `/var/log/dagu/report/draft.stdout.log` |
+| `TRACEPARENT` | Current step only, when [OpenTelemetry](/server-admin/opentelemetry#step-processes) is enabled for the DAG | W3C trace context of the step span, so a program that the step starts can create child spans. A value from the step's own `env` takes precedence. | `00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01` |
+| `TRACESTATE` | Same as `TRACEPARENT`, when the step span has a trace state | W3C trace state that goes with `TRACEPARENT`. | `vendor=value` |
 | `WEBHOOK_PAYLOAD` | Webhook-triggered runs only | JSON string containing the payload from the webhook request body. Only available when the DAG was triggered via a webhook. Left out when the payload does not fit in one environment variable; read `WEBHOOK_PAYLOAD_FILE` instead. | `{"branch":"main","commit":"abc123"}` |
 | `WEBHOOK_PAYLOAD_FILE` | Webhook-triggered runs only (v2.20 or later) | Absolute path to a private file holding the payload. Always set for webhook-triggered runs; the file is removed when the run ends. | `/tmp/dagu-webhook-1234/payload` |
 | `WEBHOOK_HEADERS` | Webhook-triggered runs only | JSON object containing the allow-listed request headers configured by `webhook.forward_headers`. Header names are lowercase and values are arrays of strings. | `{"x-github-event":["push"]}` |