feat: add trailers proto (#292)

proto/store.proto

@@ -7,6 +7,38 @@ option csharp_namespace = "Momento.Protos.Store";
 
 package store;
 
+// Found on the `err` trailer for error responses, when the error is sent by the server and it has a relevant value to set
+// {
+//   Key: "err"
+//   Values: [
+//      This is to indicate the error is coming from Momento, and not tonic or other middleware
+//    "momento_general_err",
+//      The server may or may not have tried to process this command, but it was unable
+//      to complete it either way. A resource was exhausted which prevented the request
+//      from completing.
+//    "server_is_busy",
+//      Indicates that the stored type for the key supplied isn't compatiable with the 
+//      requested operation
+//    "invalid_type",
+//      Indicates the item doesn't exist for the given key
+//    "item_not_found",
+//      Indicates the store doesn't exist
+//    "store_not_found"
+//   ]
+// },
+// Found on the `retry_disposition` trailer for error responses, when the value is known by the service
+// {
+//   Key: "retry_disposition"
+//   Values: [
+//      This rpc is safe to retry, even if it is non-idempotent. It was not executed by the server.
+//    "retryable",
+//      This rpc may be safe to retry, but it may have been applied.
+//      Non-idempotent commands should not be retried, unless configured explicitly.
+//      Idempotent commands should be considered eligible for retry.
+//    "possibly_retryable"
+//   ]
+// }
+
 service Store {
   rpc Get(_StoreGetRequest) returns (_StoreGetResponse) {}
   rpc Put(_StorePutRequest) returns (_StorePutResponse) {}
@@ -26,6 +58,11 @@ message _StoreGetRequest {
   string key = 1;
 }
 
+// This response is for when a get request concludes successfully.
+// These are some of the Errors and their corresponding GRPC status codes.
+// 1. Invalid argument was provided, value is missing -> grpc code = INVALID_ARGUMENT.  Metadata: "err" -> "momento_general_err"
+// 2. Item does not exist. grpc code = NOT_FOUND. Metadata: "err" -> "item_not_found"
+// 3. Store not found. grpc code = NOT_FOUND. Metadata: "err" -> "store_not_found"
 message _StoreGetResponse {
   _StoreValue value = 1;
 }
@@ -35,10 +72,20 @@ message _StorePutRequest {
   _StoreValue value = 2;
 }
 
+// This response is for when a put request concludes successfully.
+// These are some of the Errors and their corresponding GRPC status codes.
+// 1. Invalid argument was provided, value is missing -> grpc code = INVALID_ARGUMENT.  Metadata: "err" -> "momento_general_err"
+// 2. Store is currently busy. grpc code = UNAVAILABLE. Metadata: "err" -> "server_is_busy", "retry_disposition" -> "retryable"
+// 3. Store not found. grpc code = NOT_FOUND. Metadata: "err" -> "store_not_found"
 message _StorePutResponse { }
 
 message _StoreDeleteRequest {
   string key = 1;
 }
 
+// This response is for when a delete request concludes successfully.
+// These are some of the Errors and their corresponding GRPC status codes.
+// 1. Invalid argument was provided, value is missing -> grpc code = INVALID_ARGUMENT.  Metadata: "err" -> "momento_general_err"
+// 2. Store is currently busy. grpc code = UNAVAILABLE. Metadata: "err" -> "server_is_busy", "retry_disposition" -> "retryable"
+// 3. Store not found. grpc code = NOT_FOUND. Metadata: "err" -> "store_not_found"
 message _StoreDeleteResponse { }