chore(core): Implement disabled tracing semantics

CHANGELOG.md

@@ -8,6 +8,10 @@
 - Deprecated `ClientOptions::enable_metrics`. The option is now a no-op; metrics are always enabled. To stop sending metrics, stop calling the metrics APIs ([#1300](https://github.com/getsentry/sentry-rust/pull/1300)).
 - Deprecated [`Transaction::is_sampled`](https://docs.rs/sentry-core/0.49.2/sentry_core/struct.Transaction.html#method.is_sampled), [`Span::is_sampled`](https://docs.rs/sentry-core/0.49.2/sentry_core/struct.Span.html#method.is_sampled), and [`TransactionOrSpan::is_sampled`](https://docs.rs/sentry-core/0.49.2/sentry_core/enum.TransactionOrSpan.html#method.is_sampled). These methods cannot distinguish between an unsampled transaction or span and a deferred sampling decision when tracing is disabled ([#1293](https://github.com/getsentry/sentry-rust/pull/1293)).
 
+### Fixes
+
+- Corrected disabled tracing semantics: transactions and spans created while tracing is disabled are ignored without generating client reports, while sampling decisions received from upstream traces continue to be propagated in outgoing trace headers. An explicit `0.0` trace sample rate remains distinct from disabled tracing and continues to generate client reports for locally unsampled transactions ([#1286](https://github.com/getsentry/sentry-rust/pull/1286)).
+
 ## 0.49.1
 
 ### Fixes

sentry-core/src/performance/headers.rs

@@ -132,6 +132,11 @@ impl TracePropagationContext {
         })
     }
 
+    /// Set the `sampled` field, accepting `Option` values.
+    pub(crate) fn with_maybe_sampled(self, sampled: Option<bool>) -> Self {
+        Self { sampled, ..self }
+    }
+
     /// Attempts to construct a [`TracePropagationContext`] from the given Sentry trace header.
     ///
     /// Returns [`None`] if the header cannot be parsed.