@VisibleForTesting
  public TranslatorOutputMetricsMapFunction getOutputMetricsMF() {
    return this.outputMetricsMF;
  }
Traverse the tree of expression and validate. Only allowed predicate is conjunction of exp1 = exp2
@param rexPredicate Rex Condition
@param conjunctionList result container to pull result form recursion stack.
samza-sql/src/main/java/org/apache/samza/sql/translator/JoinTranslator.java