[LIT] Support metrics with string output (#228)

The README.md claims support for this, but it looks like that may have been broken.

Author: jroelofs

litsupport/testplan.py

@@ -141,11 +141,13 @@ def _executePlan(context, plan):
             continue
         try:
             value = lit.Test.toMetricValue(float(out))
-            context.result_metrics[metric] = value
         except ValueError:
             logging.warning(
-                "Metric reported for '%s' is not a float: '%s'", metric, out
+                "Metric reported for '%s' is not a float: '%s', treating as JSON", metric, out
             )
+            value = lit.Test.JSONMetricValue(out)
+        finally:
+            context.result_metrics[metric] = value
 
     return lit.Test.PASS