A filter for get_post_meta value

post_meta_filter.php

function dr_filter_post_meta_value( $input, int $post_id, string $meta_key, bool $single ) {

	// Don't run in admin interface
	if ( is_admin() ) {
		return $input;
	}

	// Get list of filter
	global $wp_current_filter;
	$filter_key = count( $wp_current_filter ) - 2;
	// Check if this filter is inside itself
	if ( isset( $wp_current_filter[ $filter_key ] ) && 'get_post_metadata' === $wp_current_filter[ count( $wp_current_filter ) - 2 ] ) {
		return $input;
	}

	// If my wanted meta field
	if ( 'distance' === $meta_key ) {
		// Geet the value, format and return it
		return sprintf( __( '%s km', 'textdomain' ), get_post_meta( $post_id, 'distance', true ) );
	}

	// Reeturn unmodified value
	return $input;
}
add_filter( 'get_post_metadata', 'dr_filter_post_meta_value', 10, 4 );