Link spans when rpc observation has a parent

[marcingrzejszczak]

The use case is e.g. polling mechanism in e.g. RabbitMQ.

We would create an Observation (span X) when polling starts (at which point there is no message). Next, when the message arrives we would create a child Observation. However the span Y will not be a child of the span X, because it will be populated with tracing information from the headers. What we should do though is to link span Y to span X.

cc @acogoluegnes