Maybe... Im finding that hard to answer.

This interface could be implemented correctly with A being a stateless key to a metric bound Map, in that case accumulator is simply wrong ans it is stateless and the metric is stateful. That said the design where it is an accumulator is pretty obviously better.

I think the thing thats making this hard for me to reason about is that this interface allows possibly too many implementations/interpretations. From a purely technical point all the object does is represent a context, context bound state (such as an accumulation) could be put anywhere that the metric could access it, making the object an accumulator just happens to be the cleanest solution to this general problem.

Im sorry that this doesn't really answer your question.