test: skip the serve flow-span probe when opentelemetry is absent

opentelemetry is an optional lfx extra, and the probe imports it in a
subprocess that inherits the test interpreter. On an lfx install that did
not opt in, which is what the isolated lfx CI job is, the probe errored
instead of skipping. Passed locally only because the root venv has the
extra.

Author: ogabrielluiz

src/lfx/tests/unit/cli/test_serve_app_flow_span.py

@@ -15,6 +15,13 @@
 import tempfile
 from pathlib import Path
 
+import pytest
+
+# opentelemetry is an optional lfx extra (``lfx[otel]``), and the probe below imports it in a
+# subprocess that inherits this interpreter. Without the guard this errors instead of skipping
+# on an lfx install that did not opt in, which is exactly what the isolated lfx CI job is.
+pytest.importorskip("opentelemetry")
+
 FLOW_ID = "22222222-2222-2222-2222-222222222222"
 API_KEY = "probe-key"  # pragma: allowlist secret