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.

---

But maybe there are better alternatives?

[NthPortal]

I don't think I ended up using it in any final implementation, but I was doing something like the following using SpanOps.Res#resource:

for (res <- Stream.resource(Tracer[F].span("name").resource))
yield someStream.translate(res.trace)

does this not work or keep the resource (span) open for too long or something?