feat(rust): add instructions on how to add data to a span/transaction (#12591)

* feat(rust): add instructions on how to add data to a span/transaction

* improve

* Update custom-instrumentation.mdx

* Update custom-instrumentation.mdx

* Update custom-instrumentation.mdx

Author: lcian

Diff for: docs/platforms/rust/common/tracing/instrumentation/custom-instrumentation.mdx

@@ -19,3 +19,31 @@ To capture transactions and spans customized to your organization's needs, you m
 <PlatformContent includePath="performance/concurrency" />
 
 <PlatformContent includePath="performance/connect-errors-spans" />
+
+## Adding Span & Transaction Data Attributes
+
+You can capture data attributes along with your spans and transactions. You can set data attributes when starting a span or transaction:
+
+```rust
+use serde_json::json;
+
+// Create a transaction and assign data attributes...
+let tx_ctx = sentry::TransactionContext::new("Example Transaction", "http.server");
+let transaction = sentry::start_transaction(tx_ctx);
+transaction.set_data("attribute_1", "hello".into());
+transaction.set_data("attribute_2", 42.into());
+
+// ... or create a span and assign data attributes
+let span = transaction.start_child("http.client", "Example span");
+span.set_data("is_ok", true.into());
+span.set_data("other_data", json!({ "an": "object" }));
+```
+
+Or you can add data attributes to an existing span:
+
+```rust
+let span = Hub::current().configure_scope(|scope| scope.get_span());
+if let Some(span) = span {
+    span.set_data("hello", "world".into());
+}
+```