ParentSpanId will not be set on subcalls

[me23]

Hi,

I thought i found a little bug here:

I have a Client which calls Service A. Service A will make a call to Service B.
If I look at the spans in Zipkin i have one span: Client -> Service A
and another span: Service A -> Service B.

I expected one span: Client -> Service A -> Service B.

I made a little repo to show what i mean here: https://github.com/me23/seneca-zipkin-parentid-test.

I make a fork from this repo and make a quick and dirty fix https://github.com/me23/seneca-zipkin-tracer But because I have no
experience with zipkin and writing seneca-plugins, i'm not sure if it works like expected.

May be you could look over it?!

Greetings
me23