return endpoints.MapGet(pattern, async (HttpContext context, string documentName = OpenApiConstants.DefaultDocumentName) =>
            {
                // It would be ideal to use the `HttpResponseStreamWriter` to
                // asynchronously write to the response stream here but Microsoft.OpenApi
                // does not yet support async APIs on their writers.
                // See https://github.com/microsoft/OpenAPI.NET/issues/421 for more info.
                var documentService = context.RequestServices.GetKeyedService<OpenApiDocumentService>(documentName);
                if (documentService is null)
                {
                    context.Response.StatusCode = StatusCodes.Status404NotFound;
                    context.Response.ContentType = "text/plain;charset=utf-8";
                    await context.Response.WriteAsync($"No OpenAPI document with the name '{documentName}' was found.");
                }