Limit logged headers in proxy_logging middleware

Currently all headers are logged if access_log_headers are enabled in
proxy_logging middleware. By adding the option access_log_headers_only
it is now possible to limit the logged headers to the given header values.

DocImpact
Change-Id: I0a1e40567c9eddc9bb00dd00373dc6eeb33d347c
Merge "Limit logged headers in proxy_logging middleware"