fix(tracing): add name to error event

According to the opentelemetry specification for traces [0], each span
must have a non-empty name.

[0]: https://github.com/open-telemetry/oteps/blob/main/text/trace/0059-otlp-trace-data-format.md

>  // This field is semantically required to be set to non-empty string.
>  //
>  // This field is required.
>  string name = 6;

src/extensions/tracing.rs

@@ -143,7 +143,9 @@ impl Extension for TracingExtension {
         );
         next.run(ctx, info)
             .map_err(|err| {
-                tracinglib::info!(target: "async_graphql::graphql", error = %err.message);
+                tracinglib::info!(target: "async_graphql::graphql",
+                                  error = %err.message,
+                                  "error");
                 err
             })
             .instrument(span)