zipkin

Let's support at least reading "b3" header from a single string, most commonly traceid-spanid-1
It would also be nice to support optionally writing this, especially in message providers or others with constrained environments.

Expected behavior

As discussed on openzipkin/b3-propagation#21 and first implemented here: https://github.com/openzipkin/brave/blob/master/brave/src/main/java/bra

我需要

可我在hyperf中不知道该如何使用write

@thegryfon noticed that in community axios implementations, some leak contexts and you wouldn't necessarily know until a second root trace is attempted.

here's an example scenario I think tests could cover:

there is no parent trace in scope
create a client request
create a second client request
verify that both client requests are root spans

The TracerProvider::force_flush method is part of the sdk spec but not the api spec.