perf(tracing): lower default span-export concurrency to 3

Author: smoreinis

src/agentex/lib/core/tracing/span_queue.py

@@ -25,7 +25,7 @@
 # concurrently; issuing one batch at a time caps per-pod egress at ~1/latency.
 # Sending several concurrently lets a pod keep up with span production under
 # load.  ``1`` restores the old strictly-serial behavior.
-_DEFAULT_CONCURRENCY = 8
+_DEFAULT_CONCURRENCY = 3
 # HTTP statuses worth retrying at the queue level.  These are explicit
 # backpressure / transient signals; everything else (esp. 401/403/4xx auth and
 # validation errors) is a permanent failure that re-enqueuing cannot fix.  Note