starting point for doc on how log contexts are supposed to work

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+What do I do about "Unexpected logging context" debug log-lines everywhere?
+
+<Mjark> The logging context lives in thread local storage
+<Mjark> Sometimes it gets out of sync with what it should actually be, usually because something scheduled something to run on the reactor without preserving the logging context. 
+<Matthew> what is the impact of it getting out of sync? and how and when should we preserve log context?
+<Mjark> The impact is that some of the CPU and database metrics will be under-reported, and some log lines will be mis-attributed.
+<Mjark> It should happen auto-magically in all the APIs that do IO or otherwise defer to the reactor.
+<Erik> Mjark: the other place is if we branch, e.g. using defer.gatherResults
+
+Unanswered: how and when should we preserve log context?
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