fix(webapp): record real payload size on the trigger span

When a payload is offloaded to object storage the trigger span only saw the small application/store reference, so its size attribute measured the reference, not the payload. Prefer the caller-reported pre-offload size when present so the span reflects the real payload size. Inline payloads are unchanged.

Author: 1stvamp

.server-changes/payload-size-span.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: improvement
+---
+
+Record the real pre-offload payload size on the trigger span, even when the payload was offloaded to object storage, so payload size is observable without fetching the reference.

apps/webapp/app/runEngine/concerns/payloads.server.ts

@@ -24,7 +24,11 @@ export class DefaultPayloadProcessor implements PayloadProcessor {
       );
 
       span.setAttribute("needsOffloading", needsOffloading);
-      span.setAttribute("size", size);
+      // When the caller already offloaded the payload (payloadType "application/store"), the
+      // packet here is just the small object-store reference, so `size` measures the reference,
+      // not the payload. Prefer the caller-reported pre-offload size when it's provided so the
+      // span reflects the real payload size. For inline payloads the two agree.
+      span.setAttribute("size", request.body.options?.payloadSize ?? size);
 
       if (!needsOffloading) {
         return packet;