fix(gasoline): bound traces from long-lived workflow loops

[the-company-company-staging]

Summary

Gasoline workflow loops can keep one OpenTelemetry trace ID for the full lifetime of a long-running workflow. Each loop iteration is instrumented as a child of the long-lived LoopBuilder::run / workflow span, so traces grow without a bound.

The current implementation is here:

https://github.com/rivet-dev/actors/blob/caed5835f/engine/packages/gasoline/src/builder/workflow/lupe.rs#L284

Evidence

Observed with Rivet Engine 2.3.0-rc.5 in self-hosted staging and production:

• One block-only Tempo response contained 12,205 Rivet spans over 65 minutes and required 50,389,617 inspected bytes.
• The visible iteration counters were already 7,650–7,778, showing that the trace belonged to a much older workflow parent.
• The largest operation groups were txn (2,046), txn_attempt (2,045), and four signal-listening operations (1,269 each).
• Over 36 hours, Tempo emitted 247 oversized-trace warnings from 3 trace IDs in staging and 151 warnings from 39 trace IDs in production.
• The most repeated production trace had the same shape: 12,313 spans dominated by transaction and signal-loop operations.
• These traces repeatedly hit Tempo's trace-size guard during compaction and contributed to ingester/compactor memory pressure.

The current public main still has the same inherited iteration span, and I could not find an existing issue or PR for this behavior.

Suggested fix

Start each workflow iteration as a new root trace and preserve causality by linking it to the workflow span:

let parent_span_ctx = tracing::Span::current()
    .context()
    .span()
    .span_context()
    .clone();
let iteration_span =
    tracing::info_span!(parent: None, "iteration", iteration=%previous_iteration);
iteration_span.add_link(parent_span_ctx);

let res = async {
    // existing iteration body
}
.instrument(iteration_span)
.await?;

This bounds each trace to one workflow iteration while retaining a causal link to the long-lived workflow. I have this change locally and rustfmt --check plus cargo check -p gasoline pass.

[linear-code]

ACT-6451

[Dheerajvarma1]

I'd like to work on this issue and prepare a PR based on the proposed approach. I'll investigate the current tracing behavior and add regression coverage. Please let me know if you're already planning to submit the fix.