The varnish logs should be also ingested to elasticsearch to have fine grained statistics.

It seems there is not filebeat / journalbeat supported module, but it may be possible to use a combinaison of varnishncsa [1] to generate varnish logs and filebeat to ingest them. It could also allow to have statistics on the caching effectiveness per requests.

[1] https://varnish-cache.org/docs/4.0/reference/varnishncsa.html