Assert metric reader aggregation temporality selection influence on m…

…etric persistence and start time

specification/metrics/api.md

@@ -783,8 +783,7 @@ them up) when changes occur.
 
 Note: If the values are additive (e.g. the process heap size - it makes sense to
 report the heap size from multiple processes and sum them up, so we get the
-total heap usage), use [Counter](#counter)
-or [UpDownCounter](#asynchronous-updowncounter).
+total heap usage), use [UpDownCounter](#asynchronous-updowncounter).
 
 Note: Synchronous Gauge is normally used when the measurements are exposed via a
 subscription to change events (
@@ -815,9 +814,9 @@ might consider:
 ```java
 // Java
 
-DoubleGauge componentTemperature = meter.gaugeBuilder("hw.temperature")
-    .setDescription("Temperature of hardware component")
-    .setUnit("C")
+DoubleGauge backgroundNoiseLevel = meter.gaugeBuilder("facility.noise.level")
+    .setDescription("Background noise level of rooms")
+    .setUnit("B")
     .build();
 ```
 
@@ -857,11 +856,11 @@ consider:
 
 ```java
 // Java
-Attributes memoryA = Attributes.builder().put("id", "P0 Channel A");
-Attributes memoryB = Attributes.builder().put("id", "P0 Channel B");
+Attributes roomA = Attributes.builder().put("room.id", "Rack A");
+Attributes roomB = Attributes.builder().put("room.id", "Rack B");
 
-componentTemperature.record(32.5, memoryA);
-componentTemperature.record(31.9, memoryB);
+backgroundNoiseLevel.record(4.3, roomA);
+backgroundNoiseLevel.record(2.5, roomB);
 ```
 
 ### Asynchronous Gauge

specification/metrics/data-model.md

@@ -436,10 +436,10 @@ in OTLP represents a sampled value at a given time.  A Gauge stream consists of:
 - A set of data points, each containing:
   - An independent set of Attribute name-value pairs.
   - A sampled value (e.g. current CPU temperature)
-  - A time window (of `(start, end]`) time for which the Gauge was calculated.
-    - The time interval is inclusive of the end time.
-    - Times are specified in Value is UNIX Epoch time in nanoseconds since
-        `00:00:00 UTC on 1 January 1970`
+  - A timestamp when the value was sampled (`time_unix_nano`)
+  - (optional) A timestamp (`start_time_unix_nano`) which best represents the
+    first possible moment a measurement could be recorded.  This is commonly
+    set to the timestamp when a metric collection system started.
   - (optional) a set of examplars (see [Exemplars](#exemplars)).
 
 In OTLP, a point within a Gauge stream represents the last-sampled event for a

specification/metrics/sdk.md

@@ -939,18 +939,48 @@ to be used typically with push-based metrics collection.
 
 The `MetricReader` MUST ensure that data points from OpenTelemetry
 [instruments](./api.md#instrument) are output in the configured aggregation
-temporality for each instrument kind. For synchronous instruments being output
-with Cumulative temporality, this means converting [Delta to Cumulative](supplementary-guidelines.md#synchronous-example-cumulative-aggregation-temporality)
-aggregation temporality.  For asynchronous instruments being output
-with Delta temporality, this means converting [Cumulative to
-Delta](supplementary-guidelines.md#asynchronous-example-delta-temporality) aggregation
-temporality.
+temporality for each instrument kind. For synchronous instruments with
+Cumulative aggregation temporality, this means
+converting [Delta to Cumulative](supplementary-guidelines.md#synchronous-example-cumulative-aggregation-temporality)
+aggregation temporality. For asynchronous instruments with Delta temporality,
+this means
+converting [Cumulative to Delta](supplementary-guidelines.md#asynchronous-example-delta-temporality)
+aggregation temporality.
 
 The `MetricReader` is not required to ensure data points from a non-SDK
 [MetricProducer](#metricproducer) are output in the configured aggregation
 temporality, as these data points are not collected using OpenTelemetry
 instruments.
 
+The `MetricReader` selection of `temporality` as a function of instrument kind
+influences the persistence of metric data points across collections. For
+synchronous instruments with Cumulative aggregation
+temporality, [MetricReader.Collect](#collect) MUST receive data points exposed
+in previous collections regardless of whether new measurements have been
+recorded. For synchronous instruments with Delta aggregation
+temporality, [MetricReader.Collect](#collect) MUST only receive data points with
+measurements recorded since the previous collection. For asynchronous
+instruments with Delta or Cumulative aggregation
+temporality, [MetricReader.Collect](#collect) MUST only receive data points with
+measurements recorded since the previous collection. These rules apply to all
+metrics, not just those whose [point kinds](./data-model.md#point-kinds)
+includes an aggregation temporality field.
+
+The `MetricReader` selection of `temporality` as a function of instrument kind
+influences the starting timestamp (i.e. `StartTimeUnixNano`) of metrics data
+points received by [MetricReader.Collect](#collect). For instruments with
+Cumulative aggregation temporality, successive data points received by
+successive calls to [MetricReader.Collect](#collect) MUST repeat the same
+starting timestamps (e.g. `(T0, T1], (T0, T2], (T0, T3]`). For instruments with
+Delta aggregation temporality, successive data points received by successive
+calls to [MetricReader.Collect](#collect) MUST advance the starting timestamp (
+e.g. `(T0, T1], (T1, T2], (T2, T3]`). The ending timestamp (i.e. `TimeUnixNano`)
+MUST always be equal to time the metric data point took effect, which is equal
+to when [MetricReader.Collect](#collect) was invoked. These rules apply to all
+metrics, not just those whose [point kinds](./data-model#point-kinds) includes
+an aggregation temporality field.
+See [data model temporality](./data-model.md#temporality) for more details.
+
 The SDK MUST support multiple `MetricReader` instances to be registered on the
 same `MeterProvider`, and the [MetricReader.Collect](#collect) invocation on one
 `MetricReader` instance SHOULD NOT introduce side-effects to other `MetricReader`