Default push metric exporters to use a periodic metric reader

CHANGELOG.md

@@ -28,6 +28,9 @@ release.
   ([#2317](https://github.com/open-telemetry/opentelemetry-specification/pull/2317)).
 - Clarify that expectations for user callback behavior are documentation REQUIREMENTs.
   ([#2361](https://github.com/open-telemetry/opentelemetry-specification/pull/2361)).
+- Clarify that the periodic metric reader is the default metric reader to be
+  paired with push metric exporters (OTLP, stdout, in-memory)
+  ([#2379](https://github.com/open-telemetry/opentelemetry-specification/pull/2379)).
 
 ### Logs
 

specification/metrics/sdk_exporters/in-memory.md

@@ -6,6 +6,13 @@ In-memory Metrics Exporter is a [Push Metric
 Exporter](../sdk.md#push-metric-exporter) which accumulates metrics data in the
 local memory and allows to inspect it (useful for e.g. unit tests).
 
+If a language provides a mechanism to automatically configure a
+[MetricReader](../sdk.md#metricreader) to pair with the exporter (e.g., using
+the
+[`OTEL_METRICS_EXPORTER` environment variable](../../sdk-environment-variables.md#exporter-selection)),
+by default the exporter MUST be paired with a
+[periodic exporting MetricReader](../sdk.md#periodic-exporting-metricreader).
+
 In-memory Metrics Exporter MUST support both Cumulative and Delta
 [Temporality](../datamodel.md#temporality).
 

specification/metrics/sdk_exporters/otlp.md

@@ -6,6 +6,13 @@ OTLP Metrics Exporter is a [Push Metric
 Exporter](../sdk.md#push-metric-exporter) which sends metrics via the
 [OpenTelemetry Protocol](../../protocol/README.md).
 
+If a language provides a mechanism to automatically configure a
+[MetricReader](../sdk.md#metricreader) to pair with the exporter (e.g., using
+the
+[`OTEL_METRICS_EXPORTER` environment variable](../../sdk-environment-variables.md#exporter-selection)),
+by default the exporter MUST be paired with a
+[periodic exporting MetricReader](../sdk.md#periodic-exporting-metricreader).
+
 OTLP Metrics Exporter MUST support both Cumulative and Delta
 [Temporality](../datamodel.md#temporality).
 

specification/metrics/sdk_exporters/stdout.md

@@ -6,6 +6,13 @@
 Exporter](../sdk.md#push-metric-exporter) which outputs the metrics to
 stdout/console.
 
+If a language provides a mechanism to automatically configure a
+[MetricReader](../sdk.md#metricreader) to pair with the exporter (e.g., using
+the
+[`OTEL_METRICS_EXPORTER` environment variable](../../sdk-environment-variables.md#exporter-selection)),
+by default the exporter MUST be paired with a
+[periodic exporting MetricReader](../sdk.md#periodic-exporting-metricreader).
+
 [OpenTelemetry SDK](../../overview.md#sdk) authors MAY choose the best idiomatic
 name for their language. For example, ConsoleExporter, StdoutExporter,
 StreamExporter, etc.