Logging stacktrace for the exception if MetricValidation failed for each retry (#135)

This change logs stacktrace of the failed attempt in warning level, so operator will see the missing metric even after one retry.

Author: jerry-shao

validator/src/main/java/com/amazon/aoc/helpers/RetryHelper.java

@@ -42,6 +42,7 @@ public static boolean retry(
         return true;
       } catch (Exception ex) {
         exceptionInTheEnd = ex;
+        log.warn(ex);
         if (retryCount != 0) { // don't sleep before leave this loop
           log.info(
               "retrying after {} seconds", TimeUnit.MILLISECONDS.toSeconds(sleepInMilliSeconds));