oops.... "so that people reviewing them can know what they should instead be reading"

Yeah we should do this. Marking as available so someone can pick this up.

I propose that we just mark the PLT.* histograms with PageLoad.Timing2.* equivalents as deprecated. They will disappear from the UMA dash UI.

In parallel we can send an email to chrome-team announcing that teams should migrate to PageLoad.Timing2.*, that PLT.* histograms with PageLoad.Timing2.* have been deprecated, and that we intend to deprecate all PLT.* by end of Q2, and to please contact chrome-loading@ if there are PLT.* metrics that don't page PageLoad.Timing2.* equivalents that teams are depending on.

WDYT?

I put together https://codereview.chromium.org/1837673003 as a start.

#4- sounds good to me. Should we, say, create a spreadsheet which lists up all existing PLT.* histograms which have columns for equivalent PageLoad.Timing2.* and active customers, and ask everyone to fill the latter (customers) column if anyone's actively watching it?

Also-- do we have a good data that can be shared about diffs between PLT.X and equivalent PageLoad.Timing2.X? If they look somewhat different we should probably give a heads-up about that too.

I think there are startup metrics that need thinking through too, for instance Startup.FirstWebContents.NonEmptyPaint2

Many of PLT.* metrics are broken since M47 (Issue 615781), and nobody noticed that ...
I doubt these metrics have active customers.

Issue 615781 has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7410e070f179e1e36bdb358c188c63c4a148de3c

commit 7410e070f179e1e36bdb358c188c63c4a148de3c
Author: bmcquade <bmcquade@chromium.org>
Date: Tue Aug 02 19:33:59 2016

Obsolete old PLT.* histograms in favor of PageLoad.* histograms.

In a subsequent change in a few months time, we'll remove
page_load_histograms.cc, which is responsible for logging these
histograms. We want to give a few months time where the histograms
are still logged, in case there are still users of these histograms
that we're not currently aware of.

See
https://groups.google.com/a/google.com/d/topic/chrome-metrics-team/icBD6tSvMyo/discussion
for more context.

BUG= 595742 

Review-Url: https://codereview.chromium.org/1837673003
Cr-Commit-Position: refs/heads/master@{#409272}

[modify] https://crrev.com/7410e070f179e1e36bdb358c188c63c4a148de3c/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cff018a82ea4939bff9686d55f56655e0e17cf51

commit cff018a82ea4939bff9686d55f56655e0e17cf51
Author: bmcquade <bmcquade@chromium.org>
Date: Thu Aug 04 02:35:20 2016

Obsolete unused PLT histograms.

BUG= 595742 

Review-Url: https://codereview.chromium.org/2199933002
Cr-Commit-Position: refs/heads/master@{#409707}

[modify] https://crrev.com/cff018a82ea4939bff9686d55f56655e0e17cf51/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0118e63731d7b195258c64fefc1efbc8e44ef35b

commit 0118e63731d7b195258c64fefc1efbc8e44ef35b
Author: csharrison <csharrison@chromium.org>
Date: Thu Sep 15 15:41:06 2016

Remove old prefetcher histogram code

The histograms are obsolete, and this code walks the frame tree!

BUG= 595742 

Review-Url: https://codereview.chromium.org/2332423003
Cr-Commit-Position: refs/heads/master@{#418874}

[modify] https://crrev.com/0118e63731d7b195258c64fefc1efbc8e44ef35b/chrome/renderer/page_load_histograms.cc
[modify] https://crrev.com/0118e63731d7b195258c64fefc1efbc8e44ef35b/content/public/renderer/document_state.cc
[modify] https://crrev.com/0118e63731d7b195258c64fefc1efbc8e44ef35b/content/public/renderer/document_state.h
[modify] https://crrev.com/0118e63731d7b195258c64fefc1efbc8e44ef35b/content/renderer/render_frame_impl.cc