Specify MeterProvider configurable cardinality limits

CHANGELOG.md

@@ -176,6 +176,9 @@ release.
   ([#3184](https://github.com/open-telemetry/opentelemetry-specification/pull/3184))
 - Add metric requirement levels "Required", "Recommended", and "Opt-In".
   ([#3237](https://github.com/open-telemetry/opentelemetry-specification/pull/3237))
+- Recommended cardinality limits to protect metrics pipelines against
+  excessive data production from a single instrument.
+  ([#2960](https://github.com/open-telemetry/opentelemetry-specification/pull/2960))
 
 ### Logs
 

specification/metrics/sdk.md

@@ -30,6 +30,9 @@ linkTitle: SDK
         * [Use the maximum scale for single measurements](#use-the-maximum-scale-for-single-measurements)
         * [Maintain the ideal scale](#maintain-the-ideal-scale)
   * [Observations inside asynchronous callbacks](#observations-inside-asynchronous-callbacks)
+  * [Cardinality limits](#cardinality-limits)
+    + [Synchronous instrument cardinality limits](#synchronous-instrument-cardinality-limits)
+    + [Asynchronous instrument cardinality limits](#asynchronous-instrument-cardinality-limits)
 - [Meter](#meter)
   * [Duplicate instrument registration](#duplicate-instrument-registration)
   * [Instrument name](#instrument-name)
@@ -235,6 +238,12 @@ are the inputs:
     `exemplar_reservoir` (optional) to use for storing exemplars. This should be
     a factory or callback similar to aggregation which allows different
     reservoirs to be chosen by the aggregation.
+  * **Status**: [Experimental](../document-status.md) - the
+    `aggregation_cardinality_limit` (optional) associated with the view.  This
+    should be an positive integer to be taken as a hard limit on the
+    number of data points that will be emitted during a single
+    collection by a single instrument.  See [cardinality limits](#cardinality-limits),
+    below.
 
 In order to avoid conflicts, views which specify a name SHOULD have an
 instrument selector that selects at most one instrument. For the registration
@@ -582,6 +591,62 @@ execution.
 The implementation MUST complete the execution of all callbacks for a
 given instrument before starting a subsequent round of collection.
 
+### Cardinality limits
+
+**Status**: [Experimental](../document-status.md)
+
+Views SHOULD support being configured with a cardinality limit to be
+applied to all aggregators not configured by a specific view, specified
+via `MetricReader` configuration.
+
+View configuration SHOULD support applying per-aggregation cardinality limits.
+
+The cardinality limit is taken as an exact, hard limit on the number
+of data points that can be written per collection, per aggregation.  
+Each aggregation configured view MUST NOT output more than the
+configured `aggregation_cardinality_limit` number of data points per 
+period.
+
+The RECOMMENDED default aggregation cardinality limit is 2000.
+
+An overflow attribute set is defined, containing a single attribute
+`otel.metric.overflow` having (boolean) value `true`, which is used to
+report a synthetic aggregation of the metric events that could not be
+independently aggregated because of the limit.
+
+The SDK MUST create an Aggregator with the overflow attribute set
+prior to reaching the cardinality limit and use it to aggregate events
+for which the correct Aggregator could not be created.  The maximum
+number of distinct, non-overflow attributes is one less than the
+limit, as a result.
+
+#### Synchronous instrument cardinality limits
+
+Views of synchronous instruments with cumulative aggregation
+temporality MUST continue to export the all attribute sets that were
+observed prior to the beginning of overflow.  Metric events
+corresponding with attribute sets that were not observed prior to the
+overflow will be reflected in a single data point described by (only)
+the overflow attribute.
+
+Views of synchronous instruments with delta aggregation temporality
+MAY choose an arbitrary subset of attribute sets to output to maintain
+the stated cardinality limit.
+
+Regardless of aggregation temporality, the SDK MUST ensure that every
+metric event is reflected in exactly one Aggregator, which is either
+an Aggregator associated with the correct attribute set or an
+aggregator associated with the overflow attribute set.
+
+Events MUST NOT be double-counted or dropped during an
+overflow.
+
+#### Asynchronous instrument cardinality limits
+
+Views of asynchronous instruments SHOULD prefer the first-observed
+attributes in the callback when limiting cardinality, regardless of
+aggregation temporality.
+
 ## Meter
 
 Distinct meters MUST be treated as separate namespaces for the purposes of detecting
@@ -862,6 +927,7 @@ SHOULD provide at least the following:
 * The `exporter` to use, which is a `MetricExporter` instance.
 * The default output `aggregation` (optional), a function of instrument kind.  If not configured, the [default aggregation](#default-aggregation) SHOULD be used.
 * The default output `temporality` (optional), a function of instrument kind.  If not configured, the Cumulative temporality SHOULD be used.
+* The default aggregation cardinality limit to use, a function of instrument kind.  If not configured, a default value of 2000 SHOULD be used.
 
 The [MetricReader.Collect](#collect) method allows general-purpose
 `MetricExporter` instances to explicitly initiate collection, commonly