Log correlation_id
The [correlation_id](https://docs.gitlab.com/ee/administration/logs/tracing_correlation_id.html) is a unique identifier of the current task that is being shared among all involved components, with the goal to 1. Help chase bugs 2. Be useful in metrics Here, "task" is an abstract notion: it could be an end-user request, a background job etc. It is obviously a must-have in HGitaly.
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