Get all post meta keys from the WordPress with SQL query.

get-all-meta-keys.php

<?php
if( ! function_exists( 'prefix_get_all_post_meta_keys' ) ) :
	/**
	 * Get all meta keys.
	 * 
	 * @return mixed Array of the meta keys.
	 */
	function prefix_get_all_post_meta_keys() {
		global $wpdb;

		$sql = "SELECT DISTINCT meta_key
			FROM $wpdb->postmeta
			WHERE meta_key NOT BETWEEN '_' AND '_z'
			HAVING meta_key NOT LIKE %s
			ORDER BY meta_key";

		$keys = $wpdb->get_col( $wpdb->prepare( $sql, $wpdb->esc_like( '_' ) . '%' ) );

		return $keys;
	}
endif;