Add gitlab_pages_http_time_to_write_header to pages apdex

My try on implementing gitlab-org/gitlab#335272 (closed)

Here is the test dashboard: https://dashboards.gitlab.net/dashboard/snapshot/AMHLWPdI2ClJJC1sKpLFdOydTHO6wyhh?orgId=1

But new SLI is empty there, and it's empty even I copy everything from the server SLI, maybe it's some issue the the snapshot view 🤷