Propagate correlation ID when proxying requests to another registry

Problem

In !376 (merged) we're implementing a feature to enable proxying requests that target new repositories to another registry (#218 (closed)). Currently, the only way to correlate requests across the two registry instances is by looking at the log messages that tell whether or not a request was proxied for a repository x.

This is a followup from !376 (comment 457557360).

Solution

Investigate the possibility of propagating a correlation ID across the two registry instances, linking related requests when these are proxied from the main instance to the secondary one.

This should be possible by:

• Within the proxy logic (main/left registry), extract the correlation ID from the request. We use LabKit's correlation to generate and embed a correlation ID on each incoming HTTP request (source);

• Before proxying the request to the new/right registry, add a X-Request-ID header, set to the correlation ID extracted from the previous step;

• Although we don't use it at the moment, LabKit allows us to reuse an X-Request-ID header value as correlation ID, if one is found in the incoming request (instead of generating a new one). All that we have to do is enable the WithPropagation option.

Once done, the correlation ID generated by the main/left registry should be propagated to the new/right registry. This is visible in the registry logs, where a correlation_id key/value pair is shown for each HTTP request.