Usage examples for async would be really valuable

[time4tea]

Hiya - Thanks for this library!

I'm trying to instrument an async process, and its not clear to me how to track the call through the system.

For example, plugging into 'dio' using interceptors, i can create a new span like this:

  dynamic startHttpRequest(RequestOptions options, handler) {
    var span =
        tracer.startSpan("http", context: ZoneContext.current, attributes: [
      Attribute.fromString(SemanticAttributes.httpMethod, options.method),
      Attribute.fromString(SemanticAttributes.httpHost, options.uri.host),
      Attribute.fromString(SemanticAttributes.httpScheme, options.uri.scheme),
    ]);

    print(span.spanContext.spanId);

    ZoneContext.current.withSpan(span).execute(() {
      handler.next(options);
    });
  }

and then I'm hoping to be able to close the span at the end of the request like this:

  dynamic endHttpRequest(Response<dynamic> response, handler) {
    var span = ZoneContext.current.span;

    print(span.spanContext.spanId);

    span.setAttributes([
      Attribute.fromInt(SemanticAttributes.httpStatusCode, response.statusCode!)
    ]);

    span.end();
    handler.next(response);
  }

however, the spanid at the end is the invalid span, so i guess I'm not doing the right thing with the context. Are there any examples for how to instrument something like dio?

Of course - if i get this working, it could be contributed, if that were useful.

Thank you!

[ukasz123]

@time4tea Following interceptor implementation worked for me.

import 'package:dio/dio.dart';
import 'package:opentelemetry/api.dart' as otel_api;

class TracingInterceptor implements Interceptor {
  static const _spanExtra = 'telemetry_span';

  static const _tracerName = 'HTTP-client';

  List<otel_api.Attribute> _getRequestAttributes(RequestOptions options) {
    // NO-OP
    return [];
  }

  String _getSpanName(RequestOptions options) {
    // NO-OP
    return 'http';
  }

  @override
  void onRequest(RequestOptions options, RequestInterceptorHandler handler) {
    final requestSpan =
        otel_api.globalTracerProvider.getTracer(_tracerName).startSpan(
              _getSpanName(options),
              kind: otel_api.SpanKind.client,
              attributes: _getRequestAttributes(options),
            );
    // store the span object for later
    options.extra[_spanExtra] = requestSpan;

    handler.next(options);
  }

  @override
  void onResponse(Response response, ResponseInterceptorHandler handler) {
    final requestSpan =
        response.requestOptions.extra[_spanExtra] as otel_api.Span;
    requestSpan.end();
    handler.next(response);
  }

  @override
  void onError(DioException err, ErrorInterceptorHandler handler) {
    final requestSpan = err.requestOptions.extra[_spanExtra] as otel_api.Span;
    requestSpan.recordException(err);
    requestSpan.end();
    handler.next(err);
  }
}

[time4tea]

Thank you! I will try this!

[time4tea]

I did try this and it worked very nicely.