Updates RequestHeaderSize existing filter to also take into account key size

Fixes gh-2536

docs/src/main/asciidoc/spring-cloud-gateway.adoc

@@ -1143,6 +1143,29 @@ spring:
 
 This will remove the `red` parameter before it is sent downstream.
 
+=== `RequestHeaderSize` `GatewayFilter` Factory
+
+The `RequestHeaderSize` `GatewayFilter` factory takes a `maxSize` parameter.
+It is the maximum data size allowed of the request header (incuding key and value).
+The following listing configures a `RequestHeaderSize` `GatewayFilter`:
+
+.application.yml
+====
+[source,yaml]
+----
+spring:
+  cloud:
+    gateway:
+      routes:
+      - id: requestheadersize_route
+        uri: https://example.org
+        filters:
+        - RequestHeaderSize=1000B
+----
+====
+
+This will send a status 431 if size of any request header is greater than 1000 Bytes.
+
 === The `RewritePath` `GatewayFilter` Factory
 
 The `RewritePath` `GatewayFilter` factory takes a path `regexp` parameter and a `replacement` parameter.