opentelemetry-propagator-b3 doesn't allow the receiver to make sampling decision when X-B3-Sampled header is absent

Here is the pull request: #4794

Describe your environment

OS: ALL (Ubuntu, OS X, etc.)
Python version: ALL Python 3x - Python 3.9 and later.
Package version: 1.37.0

What happened?

opentelemetry-propagator-b3 doesn't allow the receiver to make sampling decision when X-B3-Sampled header is absent.

The Zipkin/B3 protocol - https://github.com/openzipkin/b3-propagation - leaves the sampling decision up to the receiver whenever it doesn't receive the "X-B3-Sampled" HTTP header:

An accept sampling decision is encoded as X-B3-Sampled: 1 and a deny as X-B3-Sampled: 0. Absent means defer the decision to the receiver of this header.

However, opentelemetry-propagator-b3 incorrectly hard-codes absent "X-B3-Sampled" as if "X-B3-Sampled: 0", not giving the application an opportunity to make this decision. The class B3MultiFormat does not provide the means for an application to specify it's preference for the default behavior when "X-B3-Sampled" is not received

Steps to Reproduce

Instantiate B3MultiFormat and observe that the class does't offer any parameters or alternate mechanism to specify the default behavior in the event that explicit sampling state (e.g. "X-B3-Sampled") is absent from carrier.

Expected Result

A mechanism in B3MultiFormat to allow the application to decide the desired sampled state in the event that "X-B3-Sampled" is not received.

Actual Result

B3MultiFormat always drops the received "X-B3-TraceId" and "X-B3-SpanId" whenever "X-B3-Sampled" is not received.

This contradicts the Zipkin/B3 protocol specification which explicitly leaves the sampling decision up to the receiver whenever it doesn't receive the "X-B3-Sampled" HTTP header:

An accept sampling decision is encoded as X-B3-Sampled: 1 and a deny as X-B3-Sampled: 0. Absent means defer the decision to the receiver of this header.

Additional context

Would you like to implement a fix?

Yes: Here is the pull request: #4794

Tip

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Start at the B3MultiFormat class and trace how extraction handles a carrier without the X-B3-Sampled header. The change is complete when an application can choose the receiver's default sampling decision while explicit B3 sampling values retain their existing behavior; the issue references pull request #4794.