Clarify async instrument callback identity (open-telemetry#2538)

* Clarify async instrument callback identity

* fixup!

* fixup!

Co-authored-by: Joshua MacDonald <[email protected]>

Author: legendecas

CHANGELOG.md

@@ -20,6 +20,8 @@ release.
 - Add experimental `OTEL_EXPORTER_OTLP_DEFAULT_HISTOGRAM_AGGREGATION` variable for
   configuring default histogram aggregation of OTLP metric exporter
   ([#2619](https://github.com/open-telemetry/opentelemetry-specification/pull/2619)).
+- Clarify async instrument callback identity.
+  ([#2538](https://github.com/open-telemetry/opentelemetry-specification/pull/2538)).
 
 ### Logs
 

specification/metrics/api.md

@@ -366,9 +366,9 @@ 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
+Every currently registered Callback associated with a set of instruments MUST
 be evaluated exactly once during collection prior to reading data for
-that instrument.
+that instrument set.
 
 Callback functions MUST be documented as follows for the end user: