Propagate valid span context in noop tracer

[frigus02]

Change the API to propagate valid span contexts if no SDK tracer is configured and new spans are created.

Part of #181

Description

The latest specification reads:

Behavior of the API in the absence of an installed SDK

The following cases must be considered when a new Span is requested to be created, especially in relation to the requested parent SpanContext:

• A valid SpanContext is specified as the parent of the new Span: The API MUST treat this parent context as the context for the newly created Span. [...]
• No valid SpanContext is specified as the parent of the new Span: The API MUST create an non-valid [...] Span for use by the API caller. [...]

If I understand it correctly, this maps to the NoopTracer in this repo. Currently the NoopTracer always creates invalid spans.

I copied the logic from the sdk::Tracer about figuring out the correct parent span context. If this is correct, we could consider moving this logic into a function shared between the NoopTracer and the sdk::Tracer.

[TommyCpp]

Good start to address #181. Thanks!

[jtescher]

LGTM