@Override
    public String toString() {
      return value;
    }
Evaluate an expression like "{0}/({0} + {1}) * 100.0" where each positional argument represent a metrics value.
The value is converted to SpEL syntax:
 #var0/(#var0 + #var1) * 100.0
then it is evaluated in the context of the metrics parameters.
ambari-server/src/main/java/org/apache/ambari/server/state/alert/MetricSource.java