Add error tag to spans on OT endpoint middleware

[jlordiales]

Hi!
I think it would be useful for the OpenTracing endpoint middleware (https://github.com/go-kit/kit/blob/master/tracing/opentracing/endpoint.go) to inspect the error returned by the next endpoint and set the error tag to true/false accordingly (it could also optionally set the error.object log field).
In addition to just inspecting the returned error it could also check if the response struct implements endpoint.Failer and apply the same logic.

If you think this would be useful I'd be happy to create a PR, just wanted to get your thoughts first.

[peterbourgon]

@basvanbeek?

[basvanbeek]

Makes sense to add. The OpenCensus middleware already does somethjng similar with the status object and also inspects the endpoint.Failer checks.

[jlordiales]

Great, I'll create a PR this week.