Amp-wp: Cache "Error Index" count

[schlessera]

Feature description

Currently the count that is shown next to the Error Index submenu entry in the AMP admin menu is uncached and computed dynamically on every single admin page request (that includes the admin menu, so AJAX requests should not be affected).

Strangely enough, the count directly above it for the Validated URLs entry is already being cached.

We should add caching for the Error Index cache as well, and combine these into a single transient.

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Acceptance criteria

• The query counting the number of errors needing attention no longer runs on every admin page.
• The number shown in the admin menu and in the select input on the error index screen stays up to date

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

We should also add caching for the At a Glance widget while we're at it.

[westonruter]

A support topic was just raised about this as well:

Hello
in /wp-admin/admin.php?page=amp-options
I find this:

and for example in my custom plugin, all these unnecessary queries are made on the administration page

May be the same issue or an area for additional query reductions to be made.

[westonruter]

Two more related/similar support topics:

• https://wordpress.org/support/topic/very-slow-loads-and-long-query-times-on-dashboard/
• https://wordpress.org/support/topic/slow-admin-7/

[westonruter]

Related: #5336.

[schlessera]

QA passed

The values are stored in the transients and reused for both the menu construction as well as the "At a glance" widget.