v0.33.1

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Released 2026-Oct-08

• Fix a panic when a value stored with Context::with_value() calls
  Context::current() from its Drop implementation.
• Expand tracing documentation on async context propagation and child spans
  (#​828).
• Document the panic contract for observable metric callbacks (Callback and
  AsyncInstrumentBuilder::with_callback).
  (#​3640)
• Fix TraceState::from_str rejecting valid optional whitespace around list members.
  (#​3757)
• Breaking TraceState now validates list-members exactly as the
  W3C Trace Context Level 2
  specification requires. from_str, from_key_value and insert return an Err for:
  • empty keys and empty or whitespace-only values (e.g. =x, a=),
  • values containing characters outside printable ASCII (e.g. tabs), , or =;
    repeated equals signs (e.g. a==b) are no longer silently removed,
  • values with trailing spaces passed to from_key_value or insert (trailing
    spaces in a parsed header are still treated as optional whitespace),
  • duplicate keys, reported via the new TraceStateError::DuplicateKey variant.
    TraceState::delete now returns TraceState instead of Result<TraceState, TraceStateError>
TraceState's Debug output changed from TraceState(None) / TraceState(Some([..])) to TraceState([]) / TraceState([("key", "value")]).
    (#​3778)