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Update the protocol doc based on the suggested edits

Co-authored-by: Lior Lieberman <[email protected]>

Author: ahg-g

docs/proposals/003-endpoint-picker-protocol/README.md

@@ -11,10 +11,28 @@ This is the protocol between the EPP and the proxy (e.g, Envoy).
 The EPP MUST implement the Envoy
 [external processing service](https://www.envoyproxy.io/docs/envoy/latest/api-v3/service/ext_proc/v3/external_processor)protocol.
 
-For each HTTP request, the EPP MUST communicate to the proxy the picked model server endpoint, via
-adding the `x-gateway-destination-endpoint` HTTP header in the request and as an unstructured entry in the [dynamic_metadata](https://github.com/envoyproxy/go-control-plane/blob/c19bf63a811c90bf9e02f8e0dc1dcef94931ebb4/envoy/service/ext_proc/v3/external_processor.pb.go#L320) field of the ext-proc response, or otherwise return an error. The metadata entry for the picked endpoint MUST be wrapped with an outer key named `gateway-destination-endpoint.dynamic_forwarding.selected_endpoints`.
+For each HTTP request, the EPP MUST communicate to the proxy the picked model server endpoint via:
+
+1. Setting the `x-gateway-destination-endpoint` HTTP header to the selected endpoint in <ip:port> format.
+
+2. Set an unstructured entry in the [dynamic_metadata](https://github.com/envoyproxy/go-control-plane/blob/c19bf63a811c90bf9e02f8e0dc1dcef94931ebb4/envoy/service/ext_proc/v3/external_processor.pb.go#L320) field of the ext-proc response. The metadata entry for the picked endpoint MUST be wrapped with an outer key (which represents the metadata namespace) with a default of `envoy.lb`.
+
+The final metadata necessary would look like: 
+```go
+dynamicMetadata: {
+  "envoy.lb": {
+    "x-gateway-destination-endpoint": <ip:port>"  
+  }
+}
+
+
+Note:
+- If the EPP did not communicate the server endpoint via these two methods, it MUST return an error.
+- The EPP MUST not set two different values in the header and the inner response metadata value. 
+
+### Why envoy.lb namespace as a default? 
+The `envoy.lb` namesapce is a predefined namespace used for subsetting. One common way to use the selected endpoint returned from the server, is [envoy subsets](https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/load_balancing/subsets) where host metadata for subset load balancing must be placed under `envoy.lb`.
 
-The EPP MUST not set two different values in the header and the response metadata. 
 Setting different value leads to unpredictable behavior because proxies aren't guaranteed to support both paths, and so this protocol does not define what takes precedence.
 
 ## Model Server Protocol