Incorrect behavior for default unimplemented API span Tracer method

[MrAlias]

Description

Spans created prior to an SDK being set have a Tracer method will always return a noopTracer instead of a delegating tracer.

Environment

• opentelemetry-go version: v0.20.0

Steps To Reproduce

package otel

import (
	"context"
	"testing"

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

type testTracerProvider struct {
	tracers map[string]trace.Tracer
}

func newTestTracerProvider() *testTracerProvider {
	return &testTracerProvider{tracers: make(map[string]trace.Tracer)}
}

func (t *testTracerProvider) Tracer(name string, _ ...trace.TracerOption) trace.Tracer {
	tracer, ok := t.tracers[name]
	if ok {
		return tracer
	}

	tracer = &testTracer{name: name}
	t.tracers[name] = tracer
	return tracer
}

type testTracer struct {
	name string
}

func (*testTracer) Start(ctx context.Context, name string, opts ...trace.SpanOption) (context.Context, trace.Span) {
	return noop.Tracer.Start(ctx, name, opts...)
}

func TestTracerDelegation(t *testing.T) {
	defaultTracer := Tracer("test")
	_, span := defaultTracer.Start(context.Background(), "test")

	tp := newTestTracerProvider()
	tracer := tp.Tracer("test")
	SetTracerProvider(tp)
	if got, ok := span.Tracer().(*testTracer); !ok {
		t.Errorf("set tracer not delegated: got %T, want %T", span.Tracer(), tracer)
	} else if got != tracer {
		t.Errorf("incorrect tracer delegation: got %#v, want %#v", got, tracer)
	}
}

Expected behavior

The Tracer returned from the span's method should be the delegating Tracer that will create non-noop spans after the delegation.

[MrAlias]

As was pointed out in this comment it is likely a better approach to just remove the Tracer method from the Span interface and implementation. A consequence of this will be that there will not be a one-to-one mapping of OpenTracing spans, which also have this method, and OTel.