static IntegerDiff of(int before, int after) {
            return new IntegerDiff(before, after);
        }
Immutable representation of the high-level difference between two distribution configs.

<code>toString()</code> gives a human-readable summary of the diff.

See <code>DistributionDiffCalculatorTest</code> for examples of diff output.
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/DistributionDiff.java