fix: Correct spec-compliance of API Tracer.start_span

[fbogsany]

The spec states:

The API MUST return a non-recording Span with the SpanContext in the
parent Context (whether explicitly given or implicit current). If the
Span in the parent Context is already non-recording, it SHOULD be
returned directly without instantiating a new Span. If the parent
Context contains no Span, an empty non-recording Span MUST be returned
instead (i.e., having a SpanContext with all-zero Span and Trace IDs,
empty Tracestate, and unsampled TraceFlags).

With new tests that match the spec, the current implementation fails with:

non-recording span with the parent span context if the parent span is
recording [test/opentelemetry/trace/tracer_test.rb:97]: Expected
@context=#<OpenTelemetry::Trace::SpanContext:0x0000000120e17140
@trace_id="E\xC6d\xD5S+h\x8B\xC6u\xE6\xD5\ePQ\xDD",
@span_id="\xE2\xB3\x01\x99\xEC\xA0\xC1\x81",
@trace_flags=#<OpenTelemetry::Trace::TraceFlags:0x00000001052a50c0
@flags=0>,
@tracestate=#<OpenTelemetry::Trace::Tracestate:0x000000012085fdd8
@hash={}>, @remote=false>> to not be equal to
@context=#<OpenTelemetry::Trace::SpanContext:0x0000000120e17140
@trace_id="E\xC6d\xD5S+h\x8B\xC6u\xE6\xD5\ePQ\xDD",
@span_id="\xE2\xB3\x01\x99\xEC\xA0\xC1\x81",
@trace_flags=#<OpenTelemetry::Trace::TraceFlags:0x00000001052a50c0
@flags=0>,
@tracestate=#<OpenTelemetry::Trace::Tracestate:0x000000012085fdd8
@hash={}>, @remote=false>>.

The new implementation correctly passes this test (and the old tests). Shopify has been running with a similar patch in production for several years, based on a belief that the spec was incorrect - instead the implementation was wrong 🤦 .

Context: IIRC the API Tracer was written assuming that it would be used only when no SDK was present, however that is not necessarily true (anyone can create a no-op API Tracer instance even if the SDK TracerProvider is installed) and the spec for TracerConfig states:

If a Tracer is disabled, it MUST behave equivalently to a No-op Tracer.

The obvious implementation of TracerConfig would delegate to an API Tracer instance when a Tracer is disabled.

[kaylareopelle]

Thanks for catching this and sharing Shopify's patch!