Research slow GitLab Pages responses under 1Mb

From &6729 (comment 715541683):

Searching through Pages logs, for status code 200, response size < 1MB, response time > 10s requests, there are a large number of very slow requests: https://log.gprd.gitlab.net/goto/628caee025e96680635f210825fc378f

This looks unexpected, and we should research it and see if can fix anything.

Results

• Bad news: we don't include pages domain resolution time in our metrics or logs, so situation in our monitoring actually looks better, than it is. This should be fixed in gitlab-pages!674 (merged)
• Good news: I think I found the root cause for slow requests - we have very low hit-rates for two caches in GitLab Pages. I created an issue for this #351578 (closed).