Computes the percentage of values less than the given threshold.

The percentage_below function creates two local variables, total and count that are used to compute the percentage of values that fall below threshold. This rate is weighted by weights, and it is ultimately returned as percentage which is an idempotent operation that simply divides total by count.

For estimation of the metric over a stream of data, the function creates an update_op operation that updates these variables and returns the percentage.

If weights is None, weights default to 1