Problem Statement

Hi, really appreciate anyone who contributed to the OpenTracing Bridge.

The request is to make func (t *BridgeTracer) Extract(format interface{}, carrier interface{}) (ot.SpanContext, error) method able to support returning non-nil bridgeSpanContext when there is baggage only context without trace context.

Currently it requires trace context to be valid.

Similar to this in Java OpenTracing Shim: open-telemetry/opentelemetry-java#4739

There are a few reasons:

1. Inconsistent behavior in Java and Go when using the OT bridge/ shim.
2. OTel itself supports this use case (without using the OT bridge). It makes sense to make bridge support it.

Proposed Solution

Should be a simple change like below:

if !bridgeSC.otelSpanContext.IsValid() && bridgeSC.bag.Len() == 0 {
	return nil, ot.ErrSpanContextNotFound
}

Hope this request make sense. Thanks!