Any way to invoke code from VCL after a request has been serviced?

[Ryan Burn]

Hello,
>From VCL, is it possible to execute code that runs after a request has
been processed?

I'm looking into writing a module that enables Varnish for distributed
tracing using the OpenTracing project [opentracing.io]. This requires
invoking code at the beginning of a request to start a span and insert
tracing context into the request's headers and invoking code after a
request's been processed to finish the span and measure how long it
took to process.

I recently did a similar project for nginx
[github.com/rnburn/nginx-opentracing]. Nginx provides an
NGX_HTTP_LOG_PHASE [www.nginxguts.com/2011/01/phases/] that allows you
to set up handlers run after requests are serviced. Can anything
equivalent be done using VCL?

I image you could accomplish this by subscribing and regularly reading
from Varnish's shared memory log, but I'd much rather do it directly
if possible.

Thanks, Ryan