fix(span-buffer): Copy the event_id field into attributes (#94914)

Closes STREAM-294. The `event_id` field should be copied into the
`sentry.event_id` attribute, to copy what Snuba does. I'm not sure why
this field is missing, is there a specific reason? I took the liberty of
adding a ticket and putting up the PR 🙏🏻

Author: gggritso

src/sentry/spans/consumers/process_segments/convert.py

@@ -31,6 +31,7 @@
     "origin": "sentry.origin",
     "kind": "sentry.kind",
     "hash": "sentry.hash",
+    "event_id": "sentry.event_id",
 }