[docs-metrics] Customer ExemplarReservoir and View API configuration

docs/diagnostics/experimental-apis/OTEL1004.md

@@ -54,9 +54,9 @@ We are exposing these APIs experimentally for the following reasons:
 **TL;DR** We want to gather feedback on the usability of the API and for the
 need(s) in general for custom reservoirs before exposing a stable API.
 
-<!--
 ## Provide feedback
 
-Please provide feedback on [this issue](TODO) if you need stable support for
-custom `ExemplarReservoir`s.
--->
+Please provide feedback on [this
+issue](https://github.com/open-telemetry/opentelemetry-dotnet/issues/5629) if
+you need stable support for custom `ExemplarReservoir`s. The feedback will help
+inform decisions about what to expose stable and when.

docs/metrics/customizing-the-sdk/README.md

@@ -239,7 +239,7 @@ within the maximum number of buckets defined by `MaxSize`. The default
 `MaxSize` is 160 buckets and the default `MaxScale` is 20.
 
 ```csharp
-    // Change the maximum number of buckets
+    // Change the maximum number of buckets for "MyHistogram"
     .AddView(
         instrumentName: "MyHistogram",
         new Base2ExponentialBucketHistogramConfiguration { MaxSize = 40 })
@@ -251,6 +251,28 @@ by using Views.
 
 See [Program.cs](./Program.cs) for a complete example.
 
+#### Change the ExemplarReservoir
+
+> [!NOTE]
+> `MetricStreamConfiguration.ExemplarReservoirFactory` is an experimental API only
+  available in pre-release builds. For details see:
+  [OTEL1004](../../diagnostics/experimental-apis/OTEL1004.md).
+
+To set the [ExemplarReservoir](#exemplarreservoir) for an instrument, use the
+`MetricStreamConfiguration.ExemplarReservoirFactory` property on the View API:
+
+> [!IMPORTANT]
+> Setting `MetricStreamConfiguration.ExemplarReservoirFactory` alone will NOT
+  enable `Exemplar`s for an instrument. An [ExemplarFilter](#exemplarfilter)
+  MUST also be used.
+
+```csharp
+    // Use MyCustomExemplarReservoir for "MyFruitCounter"
+    .AddView(
+        instrumentName: "MyFruitCounter",
+        new MetricStreamConfiguration { ExemplarReservoirFactory = () => new MyCustomExemplarReservoir() })
+```
+
 ### Changing maximum Metric Streams
 
 Every instrument results in the creation of a single Metric stream. With Views,
@@ -421,20 +443,24 @@ and is responsible for recording `Exemplar`s. The following are the default
 reservoirs:
 
 * `AlignedHistogramBucketExemplarReservoir` is the default reservoir used for
-Histograms with buckets, and it stores at most one exemplar per histogram
-bucket. The exemplar stored is the last measurement recorded - i.e. any new
+Histograms with buckets, and it stores at most one `Exemplar` per histogram
+bucket. The `Exemplar` stored is the last measurement recorded - i.e. any new
 measurement overwrites the previous one in that bucket.
 
 * `SimpleFixedSizeExemplarReservoir` is the default reservoir used for all
-metrics except Histograms with buckets. It has a fixed reservoir pool, and
+metrics except histograms with buckets. It has a fixed reservoir pool, and
 implements the equivalent of [naive
 reservoir](https://en.wikipedia.org/wiki/Reservoir_sampling). The reservoir pool
-size (currently defaulting to 1) determines the maximum number of exemplars
+size (currently defaulting to 1) determines the maximum number of `Exemplar`s
 stored. Exponential histograms use a `SimpleFixedSizeExemplarReservoir` with a
 pool size equal to the number of buckets up to a max of `20`.
 
-> [!NOTE]
-> Currently there is no ability to change or configure `ExemplarReservoir`.
+See [Change the ExemplarReservoir](#change-the-exemplarreservoir) for details on
+how to use the View API to change `ExemplarReservoir`s for an instrument.
+
+See [Building your own
+ExemplarReservoir](../extending-the-sdk/README.md#exemplarreservoir) for details
+on how to implement custom `ExemplarReservoir`s.
 
 ### Instrumentation
 

docs/metrics/extending-the-sdk/README.md

@@ -2,7 +2,6 @@
 
 * [Building your own exporter](#exporter)
 * [Building your own reader](#reader)
-* [Building your own exemplar filter](#exemplarfilter)
 * [Building your own exemplar reservoir](#exemplarreservoir)
 * [Building your own resource detector](../../resources/README.md#resource-detector)
 * [References](#references)
@@ -72,12 +71,103 @@ to the `MeterProvider` as shown in the example [here](./Program.cs).
 
 Not supported.
 
-## ExemplarFilter
+## ExemplarReservoir
 
-Not supported.
+> [!NOTE]
+> `ExemplarReservoir` is an experimental API only available in pre-release
+  builds. For details see:
+  [OTEL1004](../../diagnostics/experimental-apis/OTEL1004.md). Please [provide
+  feedback](https://github.com/open-telemetry/opentelemetry-dotnet/issues/5629)
+  to help inform decisions about what should be exposed stable and when.
 
-## ExemplarReservoir
+Custom [ExemplarReservoir](../customizing-the-sdk/README.md#exemplarreservoir)s
+can be implemented to control how `Exemplar`s are recorded for a metric:
 
-Not supported.
+* `ExemplarReservoir`s should derive from `FixedSizeExemplarReservoir` (which
+  belongs to the [OpenTelemetry](../../../src/OpenTelemetry/README.md) package)
+  and implement the `Offer` methods.
+* The `FixedSizeExemplarReservoir` constructor accepts a `capacity` parameter to
+  control the number of `Exemplar`s which may be recorded by the
+  `ExemplarReservoir`.
+* The `virtual` `OnCollected` method is called after the `ExemplarReservoir`
+  collection operation has completed and may be used to implement cleanup or
+  reset logic.
+* The `bool` `ResetOnCollect` property on `ExemplarReservoir` is set to `true`
+  when delta aggregation temporality is used for the metric using the
+  `ExemplarReservoir`.
+* The `Offer` and `Collect` `ExemplarReservoir` methods are called concurrently
+  by the OpenTelemetry SDK. As such any state required by custom
+  `ExemplarReservoir` implementations needs to be managed using appropriate
+  thread-safety/concurrency mechanisms (`lock`, `Interlocked`, etc.).
+* Custom `ExemplarReservoir` implementations MUST NOT throw exceptions.
+  Exceptions thrown in custom implementations MAY lead to unreleased locks and
+  undefined behaviors.
+
+The following example demonstrates a custom `ExemplarReservoir` implementation
+which records `Exemplar`s for measurements which have the highest value. When
+delta aggregation temporality is used the recorded `Exemplar` will be the
+highest value for a given collection cycle. When cumulative aggregation
+temporality is used the recorded `Exemplar` will be the highest value for the
+lifetime of the process.
+
+```csharp
+class HighestValueExemplarReservoir : FixedSizeExemplarReservoir
+{
+    private readonly object lockObject = new();
+    private long? previousValueLong;
+    private double? previousValueDouble;
+
+    public HighestValueExemplarReservoir()
+        : base(capacity: 1)
+    {
+    }
+
+    public override void Offer(in ExemplarMeasurement<long> measurement)
+    {
+        if (!this.previousValueLong.HasValue || measurement.Value > this.previousValueLong.Value)
+        {
+            lock (this.lockObject)
+            {
+                if (!this.previousValueLong.HasValue || measurement.Value > this.previousValueLong.Value)
+                {
+                    this.UpdateExemplar(0, in measurement);
+                    this.previousValueLong = measurement.Value;
+                }
+            }
+        }
+    }
+
+    public override void Offer(in ExemplarMeasurement<double> measurement)
+    {
+        if (!this.previousValueDouble.HasValue || measurement.Value > this.previousValueDouble.Value)
+        {
+            lock (this.lockObject)
+            {
+                if (!this.previousValueDouble.HasValue || measurement.Value > this.previousValueDouble.Value)
+                {
+                    this.UpdateExemplar(0, in measurement);
+                    this.previousValueDouble = measurement.Value;
+                }
+            }
+        }
+    }
+
+    protected override void OnCollected()
+    {
+        if (this.ResetOnCollect)
+        {
+            lock (this.lockObject)
+            {
+                this.previousValueLong = null;
+                this.previousValueDouble = null;
+            }
+        }
+    }
+}
+```
+
+Custom [ExemplarReservoir](../customizing-the-sdk/README.md#exemplarreservoir)s
+can be configured using the View API. For details see: [Change the
+ExemplarReservoir](../customizing-the-sdk/README.md#change-the-exemplarreservoir).
 
 ## References