Leverage template enhancement output buffer for Server Timing in WP 6.9

[westonruter]

Performance Lab currently opens its own output buffer at template_redirect and sends the Server-Timing response header during template_include:

performance/plugins/performance-lab/includes/server-timing/class-perflab-server-timing.php

Lines 239 to 245 in 8368b58

	public function add_hooks(): void {
	if ( $this->use_output_buffer() ) {
	add_action( 'template_redirect', array( $this, 'start_output_buffer' ), PHP_INT_MIN );
	} else {
	add_filter( 'template_include', array( $this, 'on_template_include' ), PHP_INT_MAX );
	}
	}

In WordPress 6.9 we can now leverage the template enhancement output buffer introduced via Core-43258 (see r60936).

When the wp_start_template_enhancement_output_buffer() function exists, we can skip starting our own output buffer, and instead use the wp_template_enhancement_output_buffer filter.

Additionally, when output buffering is not desired, the new wp_before_include_template action can be used to send the Server-Timing instead of using the template_include filter.

See also:

• Reuse template enhancement output buffer in Optimization Detective for WP 6.9 #2224

[MarcinDudekDev]

Depends on #2354

[MarcinDudekDev]

I've opened #2515 to migrate the Performance Lab Server-Timing header to the WordPress 6.9 template enhancement output buffer (wp_finalized_template_enhancement_output_buffer when output buffering is enabled, wp_before_include_template otherwise).

This is the Server-Timing counterpart to the Optimization Detective migration in #2224, and similarly depends on the 6.9 minimum-version bump (#2354). Full CI is green.