Clarify that attribute requirement levels apply to instrumentation libraries

CHANGELOG.md

@@ -48,6 +48,10 @@ release.
   ([#3244](https://github.com/open-telemetry/opentelemetry-specification/pull/3244))
 - Mark `net.sock.host.port` as conditionally required.
   ([#3246](https://github.com/open-telemetry/opentelemetry-specification/pull/3246))
+- Clarify that attribute requirement levels apply to instrumentation. And that,
+  because users can transform their telemetry in a number of ways, these requirement
+  levels cannot be relied on by telemetry consumers.
+  ([#3289](https://github.com/open-telemetry/opentelemetry-specification/pull/3289))
 
 ### Compatibility
 

specification/common/attribute-requirement-level.md

@@ -19,6 +19,10 @@
 
 _This section applies to Log, Metric, Resource, and Span, and describes requirement levels for attributes defined in semantic conventions._
 
+Attribute requirement levels apply to instrumentation. Because users can transform their telemetry in a number of ways
+(e.g. metric views, span processors, and collector transformations), these requirement levels cannot be relied on by
+telemetry consumers.
+
 The following attribute requirement levels are specified:
 
 - [Required](#required)