Use custom `Serialize` implementations for metric types

While we are using the `erased-serde` crate, which should allow us to do this, ~~we don't actually have custom `Serialize` implementations for `MetricTrait` types anyway~~ (edit: we do, but we're not using them); we use the `value()` method instead. We should fix this. In particular, this would be useful for `NPVMetric`, to make it clear to the user when we are using the fallback comparison because the denominator is zero.