Datadog reporter doesn't handle 128-bit trace IDs

[dvgica]

If your application receives a 128-bit trace ID, say from an x-b3-trace-id header, and then the application tries to report a span with it, reporting will fail because the Datadog agent expects a 64-bit Int for the trace ID. I discovered this a while ago and have since worked around it using a custom KamonDataDogTranslator, but I think the failure mode was that the DD agent discarded the reported trace because the trace ID was too large - it showed up in the DD agent logs.

Regardless, it seems like DdSpanReporter should use the lower 64 bits of a 128-bit trace ID. Lower, because, as a result of testing, I've determined that the official Datadog agent does the same when encountering a 128 bit trace ID in a B3 header, and you want Kamon to be compatible with a mixed environment using the Datadog agent as well.

[dvgica]

If this sounds about right, @ivantopo, I can do the fix.

[SimunKaracic]

This sounds completely right, feel free to implement the fix! :D

[dvgica]

@SimunKaracic thanks for the confirmation. Pull request with the fix is up!