feat(devdocs): Add guidance for updating sample_rate

develop-docs/sdk/foundations/trace-propagation/index.mdx

@@ -2,14 +2,17 @@
 title: Trace Propagation
 description: How SDKs propagate trace context between services via headers, metadata, and environment variables.
 spec_id: sdk/foundations/trace-propagation
-spec_version: 1.9.0
+spec_version: 1.10.0
 spec_status: stable
 spec_depends_on:
   - id: sdk/foundations/transport/envelopes
     version: ">=1.0.0"
   - id: sdk/foundations/state-management/scopes
     version: ">=1.0.0"
 spec_changelog:
+  - version: 1.10.0
+    date: 2026-07-06
+    summary: Added Sampling Rate Propagation section
   - version: 1.9.0
     date: 2026-02-24
     summary: Consolidated into foundations/trace-propagation
@@ -248,6 +251,22 @@ A transaction's sampling decision **MUST** be passed to all of its children, inc
 
 </SpecSection>
 
+<SpecSection id="sampling-rate-propagation" status="stable" since="1.10.0">
+
+### Sampling Rate Propagation
+
+If there is an incoming `sample_rate`, it **SHOULD** be updated in the baggage if:
+- an explicit sampling decision is forced, e.g. `startTransaction(sampled: true)`
+- the `tracesSampler` is invoked
+- the `tracesSampleRate` is used
+- the SDK downsamples due to backpressure
+
+In other words, whenever the SDK's effective sampling rate overrides the incoming `sample_rate`, it needs to be updated, even if the baggage is considered immutable.
+
+Note that this rule only takes into account the effective sampling rate, i.e., the sampling rate that was actually used to make the sampling decision. If there is a `tracesSampleRate` defined, and there's an incoming sampling decision, the local `tracesSampleRate` is not the effective sampling rate -- it's ignored as the parent sampling decision takes precedence. In that case, the incoming `sample_rate` MUST NOT be updated.
+
+</SpecSection>
+
 <SpecSection id="default-propagation" status="stable" since="1.5.0">
 
 ### Default Propagation (No Tracing Config)