Allow disabling Server-Timing entirely using PERFLAB_DISABLE_SERVER_TIMING constant

[felixarntz]

Summary

Fixes #794

Note that the branch is based on the branch from #784 since it touches a lot of related code. Once that PR is merged, this one can have its base updated to trunk.

Testing

1. Set constant PERFLAB_DISABLE_SERVER_TIMING to true in your wp-config.php.
2. Ensure that Server-Timing header is not present in response.
3. Ensure that Tools > Server-Timing WP Admin screen is not available.
4. Ensure that, on a fresh install, the PL plugin's version of the object-cache.php drop-in is not placed.

Checklist

• PR has either [Focus] or Infrastructure label.
• PR has a [Type] label.
• PR has a milestone or the no milestone label.

[mukeshpanchal27]

Thanks for the PR @felixarntz. The Server-Timing API remains accessible through the handy mini plugin you've crafted: https://gist.github.com/felixarntz/63c05392dbf7d51cc7f8f4a424b1ff39.

For a comprehensive review, here's a rundown of the steps to reproduce the issue:

1. Introduce define( 'PERFLAB_DISABLE_SERVER_TIMING', true ); in your config.php file.
2. Verify the deactivation of Server-Timing API.
3. Implement the Server Timing Extra mini plugin from https://gist.github.com/felixarntz/63c05392dbf7d51cc7f8f4a424b1ff39.
4. Confirm the successful reactivation of Server-Timing API.

Correct me if i miss anything here. Thanks!

[felixarntz]

Thanks @mukeshpanchal27, good catch! In 93afb03 I implemented a fix that ensures that, even when another plugin uses the Server-Timing API, at least the header output is still being suppressed if it is generally disabled.

This way it is ensured there are no fatal errors, but the header won't be sent.

[felixarntz]

Noting that there are some unit test failures, however these are also present in trunk and stem from changes in the WordPress 6.3 release. See #796 for a PR to address them.

[mukeshpanchal27]

Thanks @felixarntz for the changes, LGTM!