src/main/java/org/jmxtrans/agent/ExpressionLanguageEngineImpl.java
public void registerExpressionEvaluator(@Nonnull String expression, @Nonnull Function evaluator) {
        functionsByName.put(expression, evaluator);
    }
public void registerExpressionEvaluator(@Nonnull String expression, @Nonnull Function evaluator) {
        functionsByName.put(expression, evaluator);
    }
Registers an expression evaluator with a static value.