`SetEqual` is available since .NET Fx 3.5 and is destined for equality
comparison of two different hash set instances. Since both instances of
the `Metrics` type has the same default comparer, the method works as
a sugar over a length check followed by iteration over one set and
probing the second one for containing the current element through
the `Contains` method.

`HashSet<T>` doesn't override `GetHashCode`. Therefore, the default
provided algorithm for reference types is used which doesn't take
fields into account as it happens for value types. That breaks the
equality contracts on Metrics.

Ideally `HashCode` type should be used for more correct computations
with less number of collisions, but that requires increasing the
minimal target framework.