Reduce cost of toString of MetricKey and MetricName

Samza runner heavily uses MetricKey.toString() and MetricName.toString() to update Samza metrics. We found that the toString methods have high CPU cost. And according to this article: https://redfin.engineering/java-string-concatenation-which-way-is-best-8f590a7d22a8, we should use "+" operator instead of String.format for string concatenation for better performance.

We do see a 10% QPS gain in nexmark queries using Samza runner with the change of using "+" operator.