Add user_guid to request logs

[will-gant]

A short explanation of the proposed change:

Add the field user_guid to request logs in CloudFoundry::Middleware::SecurityContextSetter.

An explanation of the use cases your change solves

Currently the only Cloud Controller logs which include user_guid as a field are nginx_access logs. The values of user guids appear incidentally in the log message itself in a handful of cases (about 0.6% of all logs), generally in SQL statements and errors.

The Steno logging library always adds eight fields to request logs (including the log message itself), and at present our middleware adds another three: the request_guid, b3_trace_id and b3_span_id. The latter two are added by CloudFoundry::Middleware::Zipkin, and the former by CloudFoundry::Middleware::VcapRequestId.

We think having access to the user_guid in all request logs could make it easier to debug certain kinds of issues. If you're interested in a single request, it's already possible (though a bit laborious) to trace your way back from some downstream request log to the nginx log that gives you the user_guid. But if you're seeing a pattern across many requests, there's currently no convenient way to know if they're all coming from one particular user. If every log carried the user_guid, it'd be pretty easy with tools like Kibana to do that sort of analysis.

Links to any other associated PRs

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