Amend the endpoint picker protocol to support fallbacks and subsetting

docs/proposals/004-endpoint-picker-protocol/README.md

@@ -9,27 +9,57 @@ This doc defines 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.
 
+## Endpoint Subset
+For each HTTP request, the proxy CAN communicate the subset of endpoints the EPP MUST pick from by setting an unstructured entry in the [filter metadata](https://github.com/envoyproxy/go-control-plane/blob/63a55395d7a39a8d43dcc7acc3d05e4cae7eb7a2/envoy/config/core/v3/base.pb.go#L819) field of the ext-proc request. The metadata entry for the subset list MUST be wrapped with an outer key (which represents the metadata namespace) with a default of `envoy.lb.subset_hint`.
+
+```go
+filterMetadata: {
+  "envoy.lb.subset_hint" {
+     "x-gateway-destination-endpoint-subset": [<ip:port>, <ip:port>, ...]
+  }
+}
+```
+
+If the key `x-gateway-destination-endpoint-subset` is set, the EPP MUST only select endpoints from the specified list. If none of the endpoints in the list is eligible or the list is empty, then the EPP MUST return a 429 status code. If the EPP does not select from the list, then this leads to unpredictable behavior.
+
+If the key `x-gateway-destination-endpoint-subset` is not set, then the EPP MUST select from the set defined by the `InferencePool` selector.
+
+## Destination Endpoint
 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: 
+The primary endpoint MUST be set using the key `x-gateway-destination-endpoint` as follows:
 ```go
 dynamicMetadata: {
   "envoy.lb": {
-    "x-gateway-destination-endpoint": <ip:port>"  
+    "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.
+Constraints:
+- If the EPP did not communicate the server endpoint via these two methods, it MUST return an error as follows:
+  - 503 (Serivce Unavailable) if there are no ready endpoints.
+  - 429 (Too Many Requests) if the request should be dropped (e.g., a Sheddable request, and the servers under heavy load).
 - The EPP MUST not set two different values in the header and the inner response metadata value. 
+- 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.
+
+### Destination endpoint fallback
+A single fallback endpoint CAN be set using the key `x-gateway-destination-endpoint-fallback` in the same metadata namespace as one used for `x-gateway-destination-endpoint` as follows:
 
-## 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`.
+```go
+dynamicMetadata: {
+  "envoy.lb" {
+     "x-gateway-destination-endpoint-fallback": <ip:port>
+  }
+}
+```
 
-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.
+### Why envoy.lb namespace as a default? 
+The `envoy.lb` namespace is a predefined namespace. 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`. Note that this is not related to the subsetting feature discussed above, this is an enovy implementation detail.
 
+## Matching An InferenceModel
+The model name of a request MUST match the `Sepc.ModelName` parameter of one of the `InferenceModels` referencing the `InferencePool` managed by the EPP. Otherwise, the EPP MUST return a 404 status code.