Review Prerequisites of Configuring APIcast to use OpenTracing

The prerequisites of 1.4. Configuring APIcast to use OpenTracing isn't prerequisites that are required of 3scale user.
They appear to be prerequisites that OpenTracing specification requires of the services that adopt it.

Each external request must have a unique request ID attached. This is usually in an HTTP header.

This isn't clear. Should an API client add a unique request ID to each request?

Each service must forward the request ID to other services.

This would be something that APIcast should implement.

Each service must output the request ID in the logs.

This would be something that APIcast should implement.

Each service must record additional information, such as the start and end time of the request.

This would be something that APIcast should implement.

Logs must be aggregated, and provide a way to parse them via HTTP request ID.

This would be what Jaeger is doing.

Note: If the document were to be heavily edited in relation to THREESCALE-7735, this section could be removed.