But the purpose of this module is to make these variables available from
an external client, right?
Not really available, but sampled to make statistics.
However if the external client and Nginx do not synchronize the access
to the shared memory, then the client will potentially read wrong values.
I don't think so. Each variable will be correct and is not supposed to be
related to another one (this is a false assertion if your point of view is
debugging). The script will run for example with a cycle of 10 s (as collectd
did) to give trends, no more, no less.
Then it's up to you to read the log to search for correlation if trends are
curious, it's just monitoring! But (I hope) it may be a good help for people
having many servers running Nginx.