sdk/metric: document unit-sensitivity of DefaultAggregationSelector

The default ExplicitBucketHistogram boundaries returned by
DefaultAggregationSelector are tuned for millisecond-scale measurements
and are not rescaled when an instrument is created with
metric.WithUnit("s") or any other unit. Add a godoc note so users
reading the selector can see this and point them at the knobs for
overriding the boundaries.

Fixes #5891

Signed-off-by: Ali <alliasgher123@gmail.com>

Author: alliasgher

CHANGELOG.md

@@ -30,6 +30,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - `ErrorType` in `go.opentelemetry.io/otel/semconv` now unwraps errors created with `fmt.Errorf` when deriving the `error.type` attribute. (#8133)
 - `go.opentelemetry.io/otel/sdk/log` now unwraps error chains created with `fmt.Errorf` when deriving the `error.type` attribute from errors on log records. (#8133)
 - `Set.MarshalLog` method in `go.opentelemetry.io/otel/attribute` now uses `Value.String` formatting following the [OpenTelemetry AnyValue representation for non-OTLP protocols](https://opentelemetry.io/docs/specs/otel/common/#anyvalue). (#8169)
+- Document that the default `ExplicitBucketHistogram` boundaries returned by `DefaultAggregationSelector` in `go.opentelemetry.io/otel/sdk/metric` are tuned for milliseconds and are not rescaled when an instrument is created with a different unit. (#XXXX)
 
 ### Deprecated
 

sdk/metric/reader.go

@@ -183,6 +183,14 @@ type AggregationSelector func(InstrumentKind) Aggregation
 // mapping: Counter ⇨ Sum, Observable Counter ⇨ Sum, UpDownCounter ⇨ Sum,
 // Observable UpDownCounter ⇨ Sum, Observable Gauge ⇨ LastValue,
 // Histogram ⇨ ExplicitBucketHistogram.
+//
+// The default ExplicitBucketHistogram boundaries are tuned for measurements
+// expressed in milliseconds and are not rescaled when an instrument is
+// created with a different unit (e.g. via
+// [go.opentelemetry.io/otel/metric.WithUnit]). To use different boundaries,
+// provide a custom [AggregationSelector] to your [Reader] or exporter, or
+// supply explicit per-instrument boundaries via
+// [go.opentelemetry.io/otel/metric.WithExplicitBucketBoundaries].
 func DefaultAggregationSelector(ik InstrumentKind) Aggregation {
 	switch ik {
 	case InstrumentKindCounter,