I am working on the first two. Robert, can you take over the https specific metrics (HTTPS-specific NavigationToInteractive and HTTPS-specific NavigationStartedInForeground)?  These might be useful for lite pages as well.

Where are these metrics supposed to go? UMA? If so, Previews only?

Just UMA and all loads, not necessarily just Previews.

Should be similar to the PageLoad.Clients.Scheme.Https.NavigationToParseStart metric

Don't know if I should separate these but it would probably also be helpful to have a Previews.EligibilityReason (overall metric) 

re comment#5: yeah go ahead and separate that one. It's an easy one but in a totally different place in the code 

Filed crbug/904009 for the Previews-specific ones.

Sending this to Tarun

The understat and overstat UMA metrics are in M-72. I still need to add the remaining.

Doug: Can I ask you to take this on? If not, please send it back. Note that the out of 4 metrics asked for in c#0, the first one is already done. Thanks.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c25c01ff11ea13f4c72ee0fe502181df0fe4b06b

commit c25c01ff11ea13f4c72ee0fe502181df0fe4b06b
Author: Doug Arnett <dougarnett@chromium.org>
Date: Wed Nov 28 22:34:18 2018

Two new scheme page load UMA variants for HTTPS slow page load analysis.

Adds HTTPS and HTTP scheme specific histograms for
NavigationToInteraction and NavigationStartedInForeground.

Bug:  903912 
Change-Id: I1ef870ed5cc83c6c83a703d363b85f14812942fe
Reviewed-on: https://chromium-review.googlesource.com/c/1354104
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Doug Arnett <dougarnett@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611902}
[modify] https://crrev.com/c25c01ff11ea13f4c72ee0fe502181df0fe4b06b/chrome/browser/page_load_metrics/observers/scheme_page_load_metrics_observer.cc
[modify] https://crrev.com/c25c01ff11ea13f4c72ee0fe502181df0fe4b06b/chrome/browser/page_load_metrics/observers/scheme_page_load_metrics_observer.h
[modify] https://crrev.com/c25c01ff11ea13f4c72ee0fe502181df0fe4b06b/chrome/browser/page_load_metrics/observers/scheme_page_load_metrics_observer_unittest.cc
[modify] https://crrev.com/c25c01ff11ea13f4c72ee0fe502181df0fe4b06b/tools/metrics/histograms/histograms.xml

Ok, landed the last 2 but I need some more clarity on what is wanted/expected
for Previews overall opt-out rate.

Talked offline some more.

Want these below UMA to track the user opt outs and page end reasons across all previews when shown to the user. (Either with the below names or with an ".All" suffix if that's easier)

Previews.OptOut.UserOptedOut 
Previews.PageEndReason

Will handle those two under auspices of  issue 904009