xDS test server: document rpc-behavior metadata, add hostname option (#29475 An error occurred )

* xDS test server: document rpc-behavior metadata, add fail-on option

* Update to address suggestions

* Address more existing rpc-behavior values

* Address comments

* Address comments again

doc/xds-test-descriptions.md

@@ -17,6 +17,16 @@ Server should accept these arguments:
     *   When set to true it uses XdsServerCredentials with the test server for security test cases.
         In case of secure mode, port and maintenance_port should be different.
 
+In addition, when handling requests, if the initial request metadata contains the `rpc-behavior` key, it should modify its handling of the request as follows:
+
+ - If the value matches `sleep-<int>`, the server should wait the specified number of seconds before resuming behavior matching and RPC processing.
+ - If the value matches `keep-open`, the server should never respond to the request and behavior matching ends.
+ - If the value matches `error-code-<int>`, the server should respond with the specified status code and behavior matching ends.
+ - If the value matches `success-on-retry-attempt-<int>`, and the value of the `grpc-previous-rpc-attempts` metadata field is equal to the specified number, the normal RPC processing should resume and behavior matching ends.
+ - A value can have a prefix `hostname=<string>` followed by a space. In that case, the rest of the value should only be applied if the specified hostname matches the server's hostname.
+
+The `rpc-behavior` header value can have multiple options separated by commas. In that case, the value should be split by commas and the options should be applied in the order specified. If a request has multiple `rpc-behavior` metadata values, each one should be processed that way in order.
+
 ## Client
 
 The base behavior of the xDS test client is to send a constant QPS of unary