Connect pulse (belt/inserter contents) to input on the upper right-hand combinator, and the output from the lower right hand combinator [division] is the count over the previous _S_ seconds, and only updated every _S_ seconds.
Can be used directly, or combined with an SR latch, so that assemblers can be prevented from starving the factory of precious resources (by disabling their inserter). For example, if you start trying to build nuclear power before increasing your advanced circuit supply!
Or it can be simply used to monitor belt flow at specific points in the factory.