Record single metric sample value zero

This results in incorrectly generated post-processing metric
files. Only skip it as non-existent for throughput class.

python/toolbox/metrics.py

@@ -174,7 +174,8 @@ def finish_samples():
         # All of the stored samples need to be written
         for idx in range(0, len(stored_sample)):
             if file_id in metric_data_fh.keys():
-                if stored_sample[idx]['value'] == 0 and idx not in num_written_samples.keys():
+                if (stored_sample[idx]['value'] == 0 and idx not in num_written_samples.keys() and
+                    and metric_types[idx]['desc']['class'] == "throughput"):
                     # This metric has only 1 sample and the value is 0, so it "did not do any work".  Therefore, we can just
                     # not create this metric at all.
                     # TODO: This optimization might be better if the metric source/type could opt in/out of this.