URL Metric ETag should now be required since a transitional period has now elapsed

[westonruter]

Feature Description

For #1466 we introduced the etag property for the URL Metric (#1705, #1722). At the time, we made it optional so as to avoid invaliding all existing URL Metrics. See #1466 (comment). However, now that URL Metrics have been collected for a few months, we can safely now make etag a required property of the URL Metric.

For example, this method can now return non-empty-string instead of non-empty-string|null:

performance/plugins/optimization-detective/class-od-url-metric.php

Lines 432 to 442 in d44b4c1

	/**
	* Gets ETag.
	*
	* @since 0.9.0
	*
	* @return non-empty-string|null ETag.
	*/
	public function get_etag(): ?string {
	// Since the ETag is optional for now, return null for old URL Metrics that do not have one.
	return $this->data['etag'] ?? null;
	}

Similarly here:

performance/plugins/optimization-detective/class-od-url-metric.php

Line 42 in d44b4c1

* etag?: non-empty-string,

And here:

performance/plugins/optimization-detective/class-od-url-metric.php

Lines 214 to 222 in d44b4c1

	'etag' => array(
	'description' => __( 'The ETag for the URL Metric.', 'optimization-detective' ),
	'type' => 'string',
	'pattern' => '^[0-9a-f]{32}\z',
	'minLength' => 32,
	'maxLength' => 32,
	'required' => false, // To be made required in a future release.
	'readonly' => true, // Omit from REST API.
	),

And this condition can be removed:

performance/plugins/optimization-detective/class-od-url-metric-group.php

Lines 289 to 292 in d44b4c1

	// The ETag is not populated yet, so this is stale. Eventually this will be required.
	if ( $url_metric->get_etag() === null ) {
	return false;
	}

There's probably a few other impacted bits of code.