supabase / supabase-flutter / 35872440279

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fix(supabase): keep traceparent on unsampled traces (#1870)

## What

An unsampled trace dropped every trace header. A project sampling a
fraction of its traffic therefore got no trace id in the Supabase logs
for the requests that were not sampled, and the only way out was
`respectSamplingDecision: false`, which also puts `baggage` on every
request.

`traceparent` is now sent either way, keeping its sampled flag at `00`
so nothing downstream records the request as sampled, and only
`tracestate` and `baggage`, the vendor and application data channels,
are withheld.

## Why

This is what supabase-js has always done. From `getTraceHeaders` in
`packages/core/supabase-js/src/lib/fetch.ts`:

> Unsampled traces still carry `traceparent` so Supabase logs get a
trace_id to correlate on; the flag stays `00`, so downstream tracing
never records it as sampled. `tracestate` and `baggage` (the
vendor/application data channels) are withheld on these requests.

Log correlation is the main reason to turn propagation on at all, so
losing it on unsampled traces defeated the feature for anyone not
sampling at 100 percent.

## Scope

Behavior change, no public API change. The dartdoc on
`respectSamplingDecision` is updated to describe what the flag now does.

---

Part 1 of a stack of 3:

1. **This PR**, sampling
2. #1871, `TraceContext` constructors
3. #1872, diagnostics

https://linear.app/supabase/issue/SDK-1935


<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

- **Bug Fixes**
- Unsampled traces now retain the `traceparent` header for trace
correlation while withholding `tracestate` and `baggage` by default.
- Applications can still transmit the full trace context for unsampled
traces when sampling decisions are not enforced.
- Trace headers are no longer injected when a valid `traceparent` is
unavailable.
- **Documentation**
  - Clarified how sampling decisions affect transmitted trace headers.
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