Incorrect API behavior in absence of SDK

[MrAlias]

Description

The specification states

In general, in the absence of an installed SDK, the Trace API is a "no-op" API.
This means that operations on a Tracer, or on Spans, should have no side effects and do nothing.
However, there is one important exception to this general rule, and that is related to propagation of a SpanContext:
The API MUST create a non-recording Span with the SpanContext that is in the Span in the parent Context (whether explicitly given or implicit current) or, if the parent is a non-recording Span (which it usually always is if no SDK is present), it MAY return the parent Span back from the creation method.
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).
This means that a SpanContext that has been provided by a configured Propagator will be propagated through to any child span and ultimately also Inject, but that no new SpanContexts will be created.

Spans created in a trace tree that originates with a propagator correctly implement this, they are, at the root, a non-recording span (that is stored in a context). Spans created via a Tracer prior to an SDK being set that implements the API do not. They only are no-op spans that do not handle any part of the `SpanContext.

Expected behavior

The API correctly implements the specification and will generate a non-recording span with the global Tracer prior to an SDK being set.

[MrAlias]

Validation test:

package otel

import (
	"context"
	"testing"

	"go.opentelemetry.io/otel/internal/global"
	"go.opentelemetry.io/otel/trace"
)

func TestSpanContextPropagated(t *testing.T) {
	global.ResetForTest()

	sc := trace.NewSpanContext(trace.SpanContextConfig{
		TraceID:    [16]byte{0x01},
		SpanID:     [8]byte{0x01},
		TraceFlags: trace.FlagsSampled,
	})
	ctx := trace.ContextWithSpanContext(context.Background(), sc)
	_, span := Tracer("test").Start(ctx, "test")

	if !span.SpanContext().Equal(sc) {
		t.Error("span context not propagated by default")
	}
}

[dashpole]

Does this address #877?

Based on my reading of it, it means that if I have, for example:

otelhttp.Handler.ServeHttp -> StartSpan(ctx, ...) -> otelhttp.Transport.RoundTrip

If I don't register an SDK, but do register a propagator, we would still extract context from http headers. Then, the StartSpan calls in the handler, code, and transport don't change the SpanContext in the context. Then, the context is encoded into outgoing http headers in the transport. The tracecontext headers in the incoming request should match the headers in the outgoing request, right? Or am I reading this incorrectly?

[MrAlias]

If I don't register an SDK, but do register a propagator, we would still extract context from http headers. Then, the StartSpan calls in the handler, code, and transport don't change the SpanContext in the context. Then, the context is encoded into outgoing http headers in the transport. The tracecontext headers in the incoming request should match the headers in the outgoing request, right? Or am I reading this incorrectly?

Yes, this is correct. 👍

Does this address #877?

Yeah, I think it does. @dashpole based on your understanding outlined above, should I resolve that issue with #1901 as well?

[dashpole]

Yes, you should. It would be nice to document the "Passthrough" use-case, and potentially provide an example

[MrAlias]

It would be nice to document the "Passthrough" use-case, and potentially provide an example

Tracking with #1908