The X-Qlik-Trace header contains the ID of the original client connection to the Qlik Sense Proxy Service (QPS). The same value (except for the colons, :::) is logged as ConnectionId in the QPS logs.
The header has the following format:
X-Qlik-Trace:  = <id>:::
The other services are supposed to eventually insert their corresponding connection IDs in-between the colons, as indicated below:
<QPS>:<Qlik Sense Engine Service (QES)>:<Qlik Sense Repository Service (QRS)>:<Qlik Sense Scheduler Service (QSS)>