feat: Add Tracer.non_recording_span to API

[fbogsany]

Fixes #793

There's a lot going on here. The TL;DR is that

1. we had some crufty code that was long overdue for cleanup; and
2. in order to comply with the spec, we need to make non_recording_span an instance method on Tracer (because Spans can only be created by a Tracer), and we had a bunch of places that abused Span.new (which we marked @api private) and didn't have a Tracer instance handy.

[ahayworth]

On a technical level, I think this change is 👍

On a spec level, it's a tad confusing to me so I feel like someone a little more knowledgeable about the spec should also sign-off. 😄

[fbogsany]

In light of #793 (comment) (TL;DR this method doesn't have to be an instance method on Tracer), do we prefer to:

1. Keep things the way they are in this PR (instance method on Tracer)
2. Make the method static on Tracer (leads to calls like OpenTelemetry::Trace::Tracer.non_recording_span(...)
3. Move the method to Trace (leads to calls like OpenTelemetry::Trace.non_recording_span(...))

My 2c: static method on Tracer doesn't help much; method on Trace aligns with helpers like with_span and means we don't need to sprinkle Tracer instances around.

[robertlaurin]

Move the method to Trace (leads to calls like OpenTelemetry::Trace.non_recording_span(...))

Seems like the most convenient/consistent choice.