add missing export for log_gamma

Author: daniel-habermann

bayesflow/diagnostics/metrics/__init__.py

@@ -4,3 +4,4 @@
 from .expected_calibration_error import expected_calibration_error
 from .classifier_two_sample_test import classifier_two_sample_test
 from .model_misspecification import bootstrap_comparison, summary_space_comparison
+from .log_gamma import log_gamma