Ensure we are returning no-op span according to clarified requirements

[ahayworth]

I'm not sure if we're actually doing this right or not, I haven't stared at it long enough yet: open-telemetry/opentelemetry-specification#2121

[fbogsany]

This only applies to the no-op API implementation in the absence of a SDK. Our implementation conforms to the spec:

opentelemetry-ruby/api/lib/opentelemetry/trace/tracer.rb

Lines 50 to 58 in 113e513

	def start_span(name, with_parent: nil, attributes: nil, links: nil, start_timestamp: nil, kind: nil)
	span = OpenTelemetry::Trace.current_span(with_parent)
	if span.context.valid?
	span
	else
	Span::INVALID
	end
	end

.

[fbogsany]

Further clarification here: our API Span is implicitly non-recording, so if we have a valid Span in the context, we return it.

[ahayworth]

Fantastical. 😄