Add a telemetry span over image building threads

Currently there is pretty much no structure under the toplevel
run-compose span. It's a mess of random Koji calls. This change should
group spans related to a particular image build.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>
(cherry picked from commit 661b75c6098182f652a724d3e89b913bac56e579)

pungi/util.py

@@ -498,6 +498,12 @@ def failable(
     else:
         compose.require_deliverable(variant, arch, deliverable, subvariant)
     try:
+        with tracing.span(
+            f"generate-{deliverable}",
+            variant=variant.uid,
+            arch=arch,
+            subvariant=subvariant or "",
+        ):
             yield
     except Exception as exc:
         if not can_fail: