Haystack Support with UUID

[aantono]

• Enhancement to the Haystack Codec to add support for common use of UUID as an identifier format
• Add tag for original Haystack Trace ID to be preserved for continuous stitching between multiple systems (HS -> DD -> HS)

[richardstartin]

hi @aantono we want to support this feature but merged changes yesterday which conflict with this PR. I would be happy to resolve the merge conflict and get this PR to the state where we can merge it, but don't understand the value in trying to convert the haystack trace id to a DDId. I think we could get away with storing the haystack context in the baggage. Am I missing something?

[aantono]

Hello @richardstartin ! Thanks for your offer to help resolve the PR conflicts! The reason we try to have a more "educated" conversion is to be able to have a correlation in a deterministic way between a 128-bit UUID used by Haystack (or the same would be true with B3 as well, which uses a 128-bit Hex) and a 64-bit numeric used by Datadog. The conversion is required, as otherwise there is loss of precision happening, especially if a DD instrumented application sits between 2 applications instrumented by Haystack (thus expecting the same Haystack TraceId value to be consistent on the in-coming and out-going traces).

I'm more than happy to discuss the details or brainstorm this further, if you think there is a better alternative to handling this use-case (the less complexity, the better)! ;)