Support restarting the trace with a different trace ID

[yurishkuro]

What are you trying to achieve?

When SaaS products from cloud providers participate in the tracing, the providers are often reluctant to respect customer-supplied trace ID because it can be abused by the caller (e.g. sending the same trace ID for all requests). W3C trace context explicitly allows nodes in the call graph to restart a trace with a new trace ID, and return it to the caller via the response header (currently in the design phase). However, the current OTEL SDK spec states that the tracer must always respect & reuse the inbound trace ID.

OTEL SDK may support a configurable mechanism to restart a trace. The inbound SpanContext in this case can still be recorded as a Link to external trace.

Additional context.

Previous discussion here: #998 (comment)

Alternatives.

The behavior may also be implemented purely on the instrumentation side, e.g.

spanCtx := extract(request)
var span Span
if trusted(request) {
    span = tracer.StartSpan(parent=spanCtx)
} else {
    span = tracer.StartSpan(links=[spanCtx])
}

If OTEL Spec Committee decides not to support this directly in the SDKs, then the use case should be documented in some FAQ.

[dyladan]

The response header format is not near ready for implementation. The basic fields and data format is not yet agreed on. I would strongly recommend against implementing anything based on it yet.

As far as restarting the trace, this could be a useful feature for some people and should be fairly straightforward.