fix(accumulator): Allow zero duration granularity

[Dav1dde]

A zero duration currently panics in chrono: chronotope/chrono#1474

I need support for zero duration granularity to always immediately flush after recording a value, this is very useful for (integration)-tests.

Also changes the flush check from > to >= to always flush with a zero duration granularity.

[fpacifici]

Could you provide some context on the issues you are getting in the integration tests?
You should always be able to force a flush of the accumulator in your test no matter on the granularity

[Dav1dde]

Could you provide some context on the issues you are getting in the integration tests?
You should always be able to force a flush of the accumulator in your test no matter on the granularity

The aggregation interval is configurable in Relay via config (as seconds), which ends up getting passed to rust-usage-accountant. In an integration test I want to just change Relay config while at the same time not introduce additional config just for the integration test (something a user should never configure). It is very convenient if I can set the interval to 0 and it just does the right thing, 1) no aggregation 2) immediate flush. Overall this means less code and easier parameterizable tests.