SpanBuilder: replace startResource with wrapResource

[iRevive]

Currently, there are three options to start a span:

trait SpanBuilder[F[_]] {
  def startUnmanaged: F[Span[F]]
  def start: Resource[F, Span[F]]
  def startResource[A](resource: Resource[F, A]): Resource[F, Span.Res[F]]
}

startResource bothers me a bit.

Rather than providing startResource, we can introduce wrapResource and utilize an abstract type to infer the output.

With these changes, there are only two options to start the span.
The result type is inferred automatically:

val span: Resource[F, Span[F]] = tracer.spanBuilder("test").start
val span: Resource[F, Span.Res[F, String]] = tracer.spanBuilder("test").wrapResource(Resource.pure("test")).start

The API change is the following:

// before
tracer.spanBuilder("test").startResource(Resource.pure("test")).use { case span @ Span.Res(value) =>
  ???
}

// after
tracer.spanBuilder("test").wrapResource(Resource.pure("test")).start.use { case span @ Span.Res(value) =>
  ???
}

Pros:

1. Two fixed options to start the span
2. Due to type constraints, you cannot call startUnmanaged or wrapResource on a builder that wraps a resource
3. start infers the result type automatically

Cons:

1. The API may be confusing
2. Perhaps the type inference may be tricky in some cases

P.S. The implementation of Runner is dirty. If we agree on design changes, I will provide a more elegant one.

[rossabaker]

This seems like a good idea. We need to write some real world things to find the real world inference, but I'm in favor of giving it a try.

[zmccoy]

One note is that when I first read this and saw .start I immediately went to spinning a fiber off, which isn't what this is but in the eco system I feel like that method name may make people think we're dealing with a different type than it is without closer context reading.

[iRevive]

@zmccoy valid point.

What else can we use? startSpan? There was create option at some point #37 (comment)

[zmccoy]

@iRevive Sorry for the late reply, I think startSpan is clear and not confusing.

[rossabaker]

I was working on #107, but I don't see any harm in merging this before it rots.