set a custom TraceID/SpanId

[gquintard]

Hi,

I'm not sure I'm in the right forum to ask this question, please reroute me if not.

I have a weird case, and I'm at a dead-end, so I'd love an extra pair of eyes to make sure I'm not completely foolish.
I'm trying to add telemetry to varnish. I initially went with the C++ implementation for in-process instrumentation, but varnish vmods don't have access to all the data I'd like (for example, I don't exactly know when a request is restarted, or if something blows up spectacularly).

So, I had an idea: I could use varnish's extensive logging to implement the exporter. I still need to create/propagate traceparent/tracestate HTTP headers in the vmod/vcl, but that's very little work. Now, I just need to read the JSON logs with an external tool, builds spans and just push them to a collector, right?

I thought so, but I can't find a way to force a TraceID/SpanID when creating a Span (or modifying them after creation), which is a problem since the varnish module will already be committed to a pair and will have propagated it already.

I have found IDGenerator which could be what I'm looking for. I could stash the expected TraceID/SpanID in the Context and IDGenerator could just spit them out, but it's starting to become a bit convoluted, and I think I need an adult to check my work before I go too deep in the weeds.