Add endpoint to nexus.Request (#646)

**Why?**

It is useful information.
I will add this to the server as soon as this is merged.

Author: bergundy

openapi/openapiv2.json

@@ -9326,6 +9326,10 @@
         },
         "cancelOperation": {
           "$ref": "#/definitions/v1CancelOperationRequest"
+        },
+        "endpoint": {
+          "type": "string",
+          "description": "The endpoint this request was addressed to before forwarding to the worker.\nSupported from server version 1.30.0."
         }
       },
       "description": "A Nexus request."

temporal/api/nexus/v1/message.proto

@@ -90,6 +90,10 @@ message Request {
         StartOperationRequest start_operation = 3;
         CancelOperationRequest cancel_operation = 4;
     }
+
+    // The endpoint this request was addressed to before forwarding to the worker.
+    // Supported from server version 1.30.0.
+    string endpoint = 10;
 }
 
 // Response variant for StartOperationRequest.