XD-2107&1048: --nameExpression for metrics & --incrementExpression for agg-counter

Well, keeping this state in the _module instance_ is what this PR does (see https://github.com/ericbottard/spring-xd/blob/XD-2107-1048-ontopmaster/spring-xd-analytics/src/main/java/org/springframework/xd/analytics/metrics/integration/RichGaugeHandler.java#L31) . The problem is that we need to propagate it from handler to the repo (and keeping the "received no data yet" == "counter does not exist" semantics is a good thing IMHO)