Seal public traits

[iRevive]

Closes #1029

[NthPortal]

LGTM barring the PassThroughPropagator thing

[NthPortal]

that's an interesting side effect of this change

[iRevive]

A few items remain 'unsealed':

• TextMapGetter - expected to have arbitrary instances
• TextMapPropagator - expected to have arbitrary instances
• TextMapUpdater - expected to have arbitrary instances
• KindTransformer - to be replaced soon
• AggregationTemporalitySelector - seems straightforward, not sure if we should seal it
• CardinalityLimitSelector - seems straightforward, not sure if we should seal it

---

wrt TextMap* - we can seal the interface and provide public apply. e.g.:

sealed trait TextMapGetter[A] {
  def get(carrier: A, key: String): Option[String]
  def keys(carrier: A): Iterable[String]
}

object TextMapGetter {
  def apply[A](g: (A, String) => Option[String], k: A => Iterable[String]): TextMapGetter[A] = ...
}

However, I'm unsure if it's worth the hassle, as it will affect third-party integration, which is a major inconvenience.

[NthPortal]

I don't think we should seal the TextMap* instances. if we need to evolve their APIs to add a method, it would necessitate changing the factory methods anyway, so I don't think there is any benefit