fix: correctly format fallback IP with Guid in rate limiter partition key

Author: nanotaboada

src/Dotnet.Samples.AspNetCore.WebApi/Utilities/HttpContextUtilities.cs

@@ -42,6 +42,6 @@ public static string ExtractIpAddress(HttpContext httpContext)
             return ipAddress.ToString();
         }
 
-        return httpContext.Connection.RemoteIpAddress?.ToString() ?? "unknown-{Guid.NewGuid()}";
+        return httpContext.Connection.RemoteIpAddress?.ToString() ?? $"unknown-{Guid.NewGuid()}";
     }
 }