[api] add a monotonic version number (#5012)
diff --git a/include/openthread/instance.h b/include/openthread/instance.h
index 4f69083..d32eb65 100644
--- a/include/openthread/instance.h
+++ b/include/openthread/instance.h
@@ -46,6 +46,16 @@
#endif
/**
+ * The OpenThread API monotonic version number.
+ *
+ * This number MUST increase by one each time the contents of public OpenThread API include headers change.
+ *
+ * @note This number versions both OpenThread platform and user APIs.
+ *
+ */
+#define OPENTHREAD_API_VERSION (1)
+
+/**
* @addtogroup api-instance
*
* @brief
