contrib/Shopify/sarama: Fix WrapAsyncProducer tracing support

[adw1n]

This is a continuation of now abandoned PR #487 by @HatsuneMiku3939

Closes #483

Quick recap:

1. Currently WrapAsyncProducer doesn't work at all. Like explained in contrib/Shopify/sarama: bug in AsyncProducer #483, here

   dd-trace-go/contrib/Shopify/sarama/sarama.go

   Line 207 in ecb0b80

   key := spanKey{msg.Topic, msg.Partition, msg.Offset}

   both msg.Partition and msg.Offset are always 0. Here

   dd-trace-go/contrib/Shopify/sarama/sarama.go

   Line 223 in ecb0b80

   key := spanKey{msg.Topic, msg.Partition, msg.Offset}

   msg.Partition and msg.Offset are set correctly and this line

   dd-trace-go/contrib/Shopify/sarama/sarama.go

   Line 226 in ecb0b80

   finishProducerSpan(span, msg.Partition, msg.Offset, nil)

   is never reached.
2. Kafka 0.11.0 was released on Jun 28, 2017 https://github.com/apache/kafka/releases/tag/0.11.0.0 Many open source projects default to much newer versions, e.g. Goka defaults to 2.0.0 https://github.com/lovoo/goka/blob/3d39c5afcd0a0d967c49318208e2b313bc0dfc8c/config.go#L31 As long as we document that WrapAsyncProducer requires at least 0.11.0 no one should hold a grudge. Original WrapAsyncProducer author explains the need for Kafka 0.11.0 here contrib/Shopify/sarama: add support for tracing the sarama kafka package #296 (comment)
3. Sadly fixing TestAsyncProducer tests is really problematic because of broken sarama.MockBroker for newer kafka versions. You'll see this error when you don't skip the tests:

   TestAsyncProducer/With_Successes: sarama_test.go:246: kafka: client has run out of available brokers to talk to (Is your cluster reachable?
   

   I think that practicality matters and we should fix the WrapAsyncProducer even if we can't cover it with tests. Code changes in this PR should be pretty straightforward.

[adw1n]

@dd-caleb @gbbr please could you review this PR?

[knusbaum]

@adw1n I can take a look at this today.

[knusbaum]

Aside from my minor comment, this looks basically good to me.

Thanks for linking the upstream issues for the mock tracer. I'm not happy about not being able to run the unit tests, so hopefully that can be fixed upstream soon. I'm going to spend a bit of time looking at the mock broker issue to make sure I understand it, and run some manual tests.

Seeing as a working integration with skipped tests is better than a broken integration with tests, we can still move forward even if we can't run the tests yet, provided we open an issue to track the upstream fix so we re-enable the tests when we are able.

[adw1n]

@knusbaum please could you have another look at this PR.

[knusbaum]

@adw1n Thank you for being patient with this PR.
I ran manual testing on this and it looks good.

We should follow-up on the mock broker in the future or consider adding an integration test.

I have just one minor note about the documentation string and it should be OK. If you don't have time, I can modify the doc.

As a note, there may be conflicts with #761

[adw1n]

@knusbaum I resolved the conflict you are referring to a week ago.

[knusbaum]

LGTM.
Again, thanks for being patient with this one @adw1n