Clarify async instrument callback identity

CHANGELOG.md

@@ -23,6 +23,8 @@ release.
   ([#2273](https://github.com/open-telemetry/opentelemetry-specification/pull/2273)).
 - Specify optional support for an Exponential Histogram Aggregation.
   ([#2252](https://github.com/open-telemetry/opentelemetry-specification/pull/2252))
+- Clarify async instrument callback identity.
+  ([#2538](https://github.com/open-telemetry/opentelemetry-specification/pull/2538)).
 
 ### Logs
 

specification/metrics/api.md

@@ -355,9 +355,10 @@ Where the API supports registration of `callback` functions after
 asynchronous instrumentation creation, the user MUST be able to undo
 registration of the specific callback after its registration by some means.
 
-Every currently registered Callback associated with an instrument MUST
-be evaluted exactly once during collection prior to reading data for
-that instrument.
+Every registered Callback associated with a set of instruments MUST
+be evaluated exactly once during collection prior to reading data for
+that instrument. The evaluation is identified using a combination of the
+callback itself, and the instruments set.
 
 Callback functions MUST be documented as follows for the end user: