fix: add tracing_context sampling rate docs

docs/observability/how_to_guides/sample_traces.mdx

@@ -8,6 +8,8 @@ sidebar_position: 4
 This section is relevant for those using the LangSmith SDK or LangChain, not for those logging directly with the LangSmith API.
 :::
 
+## Set global sampling rate
+
 By default, all traces are logged to LangSmith.
 To down-sample the number of traces logged to LangSmith, set the `LANGSMITH_TRACING_SAMPLING_RATE` environment variable to
 any float between `0` (no traces) and `1` (all traces).
@@ -18,3 +20,31 @@ export LANGSMITH_TRACING_SAMPLING_RATE=0.75
 ```
 
 This works for the `traceable` decorator and `RunTree` objects.
+
+## Setting different sampling rates per client
+
+You can also set sampling rates on specific `Client` instances and using the `tracing_context` context manager:
+
+```python
+from langsmith import Client, tracing_context
+
+# Create clients with different sampling rates
+client_1 = Client(tracing_sampling_rate=0.5)  # 50% sampling
+client_2 = Client(tracing_sampling_rate=0.25)  # 25% sampling
+client_no_trace = Client(tracing_sampling_rate=0.0)  # No tracing
+
+# Use different sampling rates for different operations
+with tracing_context(client=client_1):
+    # Your code here - will be traced with 50% sampling rate
+    agent_1.invoke(...)
+
+with tracing_context(client=client_2):
+    # Your code here - will be traced with 25% sampling rate
+    agent_1.invoke(...)
+
+with tracing_context(client=client_no_trace):
+    # Your code here - will not be traced
+    agent_1.invoke(...)
+```
+
+This allows you to control sampling rates at the operation level.