fix: align request ID to standard x-request-id with backward compatibility

this adds x-request-id support in both headers and trailers:

- RequestIDHeader: new header constant for HTTP/1.1 clients that can't
  easily read trailers through the gateway

- RequestID: stays as a trailer to maintain gRPC retry policy
  compatibility (see cefe49ab for why trailers matter)

- RequestIDLegacy: keeps the old io.spicedb.respmeta.requestid key
  around so existing clients don't break during transition

Author: Verolop

pkg/responsemeta/responsemeta.go

@@ -13,8 +13,9 @@ import (
 type ResponseMetadataHeaderKey string
 
 const (
-	// RequestID is the key in the response header metadata for the request's tracking ID, if any.
-	RequestID ResponseMetadataHeaderKey = "x-request-id"
+	// RequestIDHeader is the key in the response header metadata for the request's tracking ID.
+	// Provided for HTTP gateway compatibility where trailers are not easily accessible.
+	RequestIDHeader ResponseMetadataHeaderKey = "x-request-id"
 
 	// ServerVersion is the key in the response header metadata holding the version of the server
 	// handling the API request, if requested via a request header.
@@ -25,6 +26,14 @@ const (
 type ResponseMetadataTrailerKey string
 
 const (
+	// RequestID is the key in the response trailer metadata for the request's tracking ID.
+	// Uses trailer to maintain gRPC retry policy compatibility (see gRPC proposal A6).
+	RequestID ResponseMetadataTrailerKey = "x-request-id"
+
+	// RequestIDLegacy is the legacy key in the response trailer metadata for backward compatibility.
+	// Kept to avoid breaking existing clients during transition period.
+	RequestIDLegacy ResponseMetadataTrailerKey = "io.spicedb.respmeta.requestid"
+
 	// DispatchedOperationsCount is the key in the response trailer metadata for
 	// the number of dispatched operations that were needed to perform the overall
 	// API call.