Add methods to increment/decrement by a given value

We need this extension of the CounterService in our own [Spring Boot Starter for Batch](https://github.com/codecentric/spring-boot-starter-batch-web), because sometimes we increment/decrement the counters by a value greater than 1. Actually we have [extended](https://github.com/codecentric/spring-boot-starter-batch-web/blob/master/src/main/java/de/codecentric/batch/metrics/ExtendedCounterService.java) the Interface.