Measurement Cardinality < 0

Under some circumstances, measurement cardinality (as shown in debug vars) can be less than 0. This does not appear to indicate an underlying issue, rather that of two estimates being used to calculate the number (estimate(measurements) - estimate(tombstones)). If these two measurements are close to each other, you could end up with a negative number. Result should be clamped to avoid wacky answers like negative numbers.