add documentation

Author: giortzisg

docs/platforms/go/common/tracing/instrumentation/custom-instrumentation/index.mdx

@@ -68,6 +68,19 @@ Or you can add data attributes to an existing transaction or span:
 		})
 ```
 
+Or you can update existing transaction and span data by:
+
+```go
+			if d := transaction.Data["dataAttr1"]; d != nil {
+    		transaction.SetData("dataAttr1", d.(int)+42)
+    	}
+
+    	if d := span.Data["dataAttr1"]; d != nil {
+    		span.SetData("dataAttr1", d.(int)+42)
+    	}
+		})
+```
+
 To attach data attributes to the transaction and all its spans, you can use `BeforeSendTransaction`:
 
 ```go