TODO: overview of writing custom integrations
9.1 Writing Custom Route Predicate Factories
TODO: document writing Custom Route Predicate Factories
9.2 Writing Custom GatewayFilter Factories
TODO: document writing Custom GatewayFilter Factories
9.3 Writing Custom Global Filters
TODO: document writing Custom Global Filters
9.4 Writing Custom Route Locators and Writers
TODO: document writing Custom Route Locators and Writers