docs: updates for #2844 changes (#2845)

Author: seratch

docs/tracing.md

@@ -44,6 +44,57 @@ By default, the trace is named "Agent workflow". You can set this name if you us
 
 In addition, you can set up [custom trace processors](#custom-tracing-processors) to push traces to other destinations (as a replacement, or secondary destination).
 
+## Long-running workers and immediate exports
+
+The default [`BatchTraceProcessor`][agents.tracing.processors.BatchTraceProcessor] exports traces
+in the background every few seconds, or sooner when the in-memory queue reaches its size trigger,
+and also performs a final flush when the process exits. In long-running workers such as Celery,
+RQ, Dramatiq, or FastAPI background tasks, this means traces are usually exported automatically
+without any extra code, but they may not appear in the Traces dashboard immediately after each job
+finishes.
+
+If you need an immediate delivery guarantee at the end of a unit of work, call
+[`flush_traces()`][agents.tracing.flush_traces] after the trace context exits.
+
+```python
+from agents import Runner, flush_traces, trace
+
+
+@celery_app.task
+def run_agent_task(prompt: str):
+    try:
+        with trace("celery_task"):
+            result = Runner.run_sync(agent, prompt)
+        return result.final_output
+    finally:
+        flush_traces()
+```
+
+```python
+from fastapi import BackgroundTasks, FastAPI
+from agents import Runner, flush_traces, trace
+
+app = FastAPI()
+
+
+def process_in_background(prompt: str) -> None:
+    try:
+        with trace("background_job"):
+            Runner.run_sync(agent, prompt)
+    finally:
+        flush_traces()
+
+
+@app.post("/run")
+async def run(prompt: str, background_tasks: BackgroundTasks):
+    background_tasks.add_task(process_in_background, prompt)
+    return {"status": "queued"}
+```
+
+[`flush_traces()`][agents.tracing.flush_traces] blocks until currently buffered traces and spans are
+exported, so call it after `trace()` closes to avoid flushing a partially built trace. You can skip
+this call when the default export latency is acceptable.
+
 ## Higher level traces
 
 Sometimes, you might want multiple calls to `run()` to be part of a single trace. You can do this by wrapping the entire code in a `trace()`.