Add documentation for forwarding headers (#5662)

* Add documentation for forwarding headers

* Add an example

Author: Tabrizian

docs/protocol/extension_parameters.md

@@ -85,3 +85,18 @@ Content-Length: <xx>
 The `parameters` field in the
 ModelInferRequest message can be used to send custom parameters.
 
+## Forwarding HTTP/GRPC Headers as Parameters
+
+Triton can forward HTTP/GRPC headers as inference request parameters. By 
+specifying a regular expression in `--http-header-forward-pattern` and
+`--grpc-header-forward-pattern`,
+Triton will add the headers that match with the regular experession as request
+parameters. All the forwarded headers will be added as a parameter with string
+value. For example to forward all the headers that start with 'PREFIX_' from 
+both HTTP and GRPC, you should add `--http-header-forward-pattern PREFIX_.*
+--grpc-header-forward-pattern PREFIX_.*` to your `tritonserver` command.
+
+The forwarded headers can be accessed using the
+[Python](https://github.com/triton-inference-server/python_backend#inference-request-parameters)
+or C Backend APIs as inference request parameters.
+