Context API improvements

[mwear]

As mentioned in #201 there are some situations where our current APIs fall short. This PR makes some improvements to our existing APIs to make some situations easier to handle. Below is a summary of the changes with some examples demonstrating the improvements.

I have a span, but I want it in a context

We have discussed how it's sometimes covenient to have a handle on span, where in other situations, you need to have a handle on a context containing a span. The Go SIG solved this by having API methods return both a span and a context. While multiple return values are not the best choice for Ruby, we can yield multiple parameters to blocks for Tracer#in_span and Tracer#with_span to achieve the same effect.

Before:

Tracer.in_span('my-span') { ctx = Context.current }

Now:

Tracer.in_span('my-span') { |span, ctx| ...}

I have a context, but I want a span

Tracer#current_span takes an optional context parameter to read the current span from a context other than Context.current.

Before:

span = context[OpenTelemetry::Trace::Propagation::ContextKeys.current_span_key]

Now:

span = Tracer.current_span(context)

Context.with_* methods yield context

The various Context.with_* now yield context arguments to blocks they execute.

Before:

# would only yield values
Context.with_values(k1 => 'v1', k2 => 'v2') { |values| ctx = Context.current }

Now:

# yields context and values
Context.with_values(k1 => 'v1', k2 => 'v2') { |ctx, values| ... }

There might be more scenarios we need utilities for, but this adds convenience to already existing methods for some use cases that were previously more challenging than they should be.