ActivityRequestHeaderRetrievable.
In the past HeaderRetrievable have only existed for 'incoming requests' and the plugins have a reliance on this implicit semantics.
So using this interface to retrieve 'outgoing' request headers would break the plugins that rely on this semantics.
The Activity/DownstreamHeaderRetrievable makes it explicit the direction of the headers and is a more concrete public contract.
A composable interface that disco events can implement if consumers of those events should be able to
retrieve the headers of the request associated with the event.@Deprecated public interface HeaderRetrievable extends ActivityRequestHeaderRetrievable
getAllHeaders, getFirstHeader, getHeaders