comment on two aliases

CHANGELOG.md

@@ -11,7 +11,6 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 ## Changed
 
 - Skip links with invalid span context. (#2275)
-
 - Metrics API cleanup. The `metric/sdkapi` package has been created to relocate the API-to-SDK interface:
   - The following interface types simply moved from `metric` to `metric/sdkapi`: `Descriptor`, `MeterImpl`, `InstrumentImpl`, `SyncImpl`, `BoundSyncImpl`, `AsyncImpl`, `AsyncRunner`, `AsyncSingleRunner`, and `AsyncBatchRunner`
   - The following struct types moved and are replaced with type aliases, since they are exposed to the user: `Observation`, `Measurement`.

metric/metric.go

@@ -51,11 +51,17 @@ func WrapMeterImpl(impl sdkapi.MeterImpl) Meter {
 // Measurement is used for reporting a synchronous batch of metric
 // values. Instances of this type should be created by synchronous
 // instruments (e.g., Int64Counter.Measurement()).
+//
+// Note: This is an alias because it is a first-class member of the
+// API but is also part of the lower-level sdkapi interface.
 type Measurement = sdkapi.Measurement
 
 // Observation is used for reporting an asynchronous  batch of metric
 // values. Instances of this type should be created by asynchronous
 // instruments (e.g., Int64GaugeObserver.Observation()).
+//
+// Note: This is an alias because it is a first-class member of the
+// API but is also part of the lower-level sdkapi interface.
 type Observation = sdkapi.Observation
 
 // RecordBatch atomically records a batch of measurements.