Clarify that attribute requirement levels apply to instrumentation libraries (#3289)

Based on discussion in semconv stability WG

Closes #3283

## Changes

Clarifies that attribute requirement levels apply to instrumentation.
And that, 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.

---------

Co-authored-by: Liudmila Molkova <[email protected]>
Co-authored-by: Bogdan Drutu <[email protected]>

Author: 3 people

specification/common/attribute-requirement-level.md

@@ -19,6 +19,8 @@
 
 _This section applies to Log, Metric, Resource, and Span, and describes requirement levels for attributes defined in semantic conventions._
 
+Attribute requirement levels apply to the [instrumentation library](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/glossary.md#instrumentation-library).
+
 The following attribute requirement levels are specified:
 
 - [Required](#required)