[ci] Collapse list of agents in annotiation (#186009)

Collapses the list of agents at the top of the Buildkite UI. Thanks to
`@brianseeders` for the fix.

Before:
https://buildkite.com/elastic/kibana-on-merge/builds/46264#annotation-ctx-gobld-metrics

.buildkite/scripts/lifecycle/pre_command.sh

@@ -22,3 +22,11 @@ if [[ "$(curl -is metadata.google.internal || true)" ]]; then
   echo "gcloud compute ssh --tunnel-through-iap --project elastic-kibana-ci --zone \"$(curl -sH Metadata-Flavor:Google http://metadata.google.internal/computeMetadata/v1/instance/zone)\" \"$(curl -sH Metadata-Flavor:Google http://metadata.google.internal/computeMetadata/v1/instance/name)\""
   echo ""
 fi
+
+if [[ "${BUILDKITE_LABEL:-}" == *"Run Dynamic Pipeline"* || "${BUILDKITE_LABEL:-}" == *"Upload Pipeline"* ]]; then
+  cat << EOF | buildkite-agent annotate --context "ctx-gobld-metrics" --style "info"
+<details>
+
+<summary>Agent information from gobld</summary>
+EOF
+fi