fs2 Stream instrumentation

[iRevive]

Currently, it's hard to instrument the fs2 stream properly.

Perhaps we can follow the trace4cats route and encode the stream as WriterT[Stream[F, *], Span[F], A]:

type TracedStream[F[_]] = WriterT[Stream[F, *], Span[F], A]

implicit class StreamOps[F[_], A](private val s: Stream[F, A]) {
  def traced(name: String)(implicit A: Applicative[F], T: Tracer[F]): TracedStream[F, A] =
    Stream.bracket(Tracer[F].span(name).startUnmanaged)(_.end).flatMap { span =>
      s.evalMapChunk(a => Applicative[F].pure((span, a)))
    }
}

implicit class TracedStreamOps[F[_]](private val s: TracedStream[F, A]] {
  def evalMapTraced(name: String, attributes: Attributes): TracedStream[F, A] = ...
}

And a bunch of overloaded syntax methods.

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But maybe there are better alternatives?