query_by_custom_fields.php

add_filter( 'json_query_vars', 'slug_allow_meta' );
function slug_allow_meta( $valid_vars ) {
    $valid_vars = array_merge( $valid_vars, array( 'meta_key', 'meta_value' ) );
    return $valid_vars;
}

//
$url = 'posts?filter[meta_key]=foo&filter[meta_value]=bar';