Disallow addition of attributes to existing metrics

specification/versioning-and-stability.md

@@ -172,10 +172,14 @@ and published
 For details see [how OpenTelemetry Schemas are
 published](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/schemas/overview.md#opentelemetry-schema).
 
-In addition to the 3 types of changes described above, adding conventions for
-new attributes, resources, metrics, spans, span events or log records is always
-allowed. Such changes do not need to be described (and are not described) by
-schema files.
+In addition to the 3 types of changes described above there are certain types
+that are always allowed. Such changes do not need to be described (and are not
+described) by schema files. Here is the list of such changes:
+
+- Adding new attributes to the existing semantic conventions for resources,
+  spans, span events or log records.
+- Adding semantic conventions for new types of resources, spans, span events,
+  metrics or log records.
 
 Any other changes to semantic conventions are currently prohibited. Other types
 of changes MAY be introduced in the future versions of this specification. This