Call `add_user_meta()` when adding metadata to a new user

[johnbillion]

When a user is created via wp_insert_user(), several meta fields are added to the user (including nickname, first name, last name, and various preferences). These are all added using update_user_meta() instead of add_user_meta(). This means an unnecessary database query is performed for each meta field to first check whether a matching meta row exists.

By switching to add_user_meta(), 11 database queries are saved by default when inserting a user.

Todo

• Provide list of before/after queries when inserting a user
• Confirm whether test coverage is sufficient
• Provide steps for testing
• Investigate the same for posts, comments, and terms in follow up tickets

This saves an unnecessary database query to look up an existing row that does not exist.

Trac ticket: TBD

[johnbillion]

Testing steps:

• Call wp_insert_user() with at least user_login and user_email provided.
• Confirm that all default meta fields are correctly inserted (several have an empty value by default):
  • nickname
  • first_name
  • last_name
  • description
  • rich_editing
  • syntax_highlighting
  • comment_shortcuts
  • admin_color
  • use_ssl
  • show_admin_bar_front
  • locale
• Repeat with a meta_input field included to include custom user meta fields

The test_wp_insert_user_with_meta() test covers this functionality and is sufficient for confirming that user meta data is saved when a user is created.

Database queries:

Before this change: 54
After this change: 43

As expected, this saves 11 database queries, one for each of the default meta values stored with every user.

[johnbillion]

In 56478:

Users: Call add_user_meta() instead of update_user_meta() when adding metadata to a new user.

This improves the performance of inserting users as it removes one unnecessary SELECT query for every row of metadata inserted.

Props swissspidy, spacedmonkey, johnbillion

Fixes #59212

https://core.trac.wordpress.org/changeset/56478

[lkraav]

Lgtm! Did you discover any clarity on why this was implemented with update_user_meta() in the first place?