Add basic ExemplarFilter doc. (#4246)

Co-authored-by: Utkarsh Umesan Pillai <utpilla@microsoft.com>

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

@@ -447,6 +447,15 @@ using var meterProvider = Sdk.CreateMeterProviderBuilder()
 > As of today, there is no separate toggle for enable/disable Exemplar
 feature. It can be turned off by using `AlwaysOffExemplarFilter`.
 
+If the built-in `ExemplarFilter`s are not meeting the needs, one may author
+custom `ExemplarFilter` as shown
+[here](../extending-the-sdk/README.md#exemplarfilter). A custom filter, which
+eliminates all un-interesting measurements from becoming Exemplar is a
+recommended way to control performance overhead associated with collecting
+Exemplars. See
+[benchmark](../../../test/Benchmarks/Metrics/ExemplarBenchmarks.cs) to see how
+much impact can `ExemplarFilter` have on performance.
+
 #### ExemplarReservoir
 
 `ExemplarReservoir` receives the measurements sampled in by the `ExemplarFilter`

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

@@ -2,7 +2,8 @@
 
 * [Building your own exporter](#exporter)
 * [Building your own reader](#reader)
-* [Building your own exemplar](#exemplar)
+* [Building your own exemplar filter](#exemplarfilter)
+* [Building your own exemplar reservoir](#exemplarreservoir)
 * [References](#references)
 
 ## Exporter
@@ -70,7 +71,48 @@ to the `MeterProvider` as shown in the example [here](./Program.cs).
 
 Not supported.
 
-## Exemplar
+## ExemplarFilter
+
+OpenTelemetry .NET SDK has provided the following built-in `ExemplarFilter`s:
+
+* [AlwaysOnExemplarFilter](../../../src/OpenTelemetry/Metrics/Exemplar/AlwaysOnExemplarFilter.cs)
+* [AlwaysOffExemplarFilter](../../../src/OpenTelemetry/Metrics/Exemplar/AlwaysOffExemplarFilter.cs)
+* [TraceBasedExemplarFilter](../../../src/OpenTelemetry/Metrics/Exemplar/TraceBasedExemplarFilter.cs)
+
+Custom exemplar filters can be implemented to achieve filtering based on other criterion:
+
+* `ExemplarFilter` should derive from `OpenTelemetry.ExemplarFilter` (which
+  belongs to the [OpenTelemetry](../../../src/OpenTelemetry/README.md) package)
+  and implement the `ShouldSample` method.
+
+One example is a filter, which filters all measurements of value lower
+than given threshold is given below. Such a filter prevents any measurements
+below the given threshold from ever becoming a `Exemplar`. Such filters could
+also incorporate the `TraceBasedExemplarFilter` condition as well, as storing
+exemplars for non-sampled traces may be undesired.
+
+```csharp
+public sealed class HighValueFilter : ExemplarFilter
+{
+    private readonly double maxValue;
+
+    public HighValueFilter(double maxValue)
+    {
+        this.maxValue = maxValue;
+    }
+    public override bool ShouldSample(long value, ReadOnlySpan<KeyValuePair<string, object>> tags)
+    {
+        return Activity.Current?.Recorded && value > this.maxValue;
+    }
+
+    public override bool ShouldSample(double value, ReadOnlySpan<KeyValuePair<string, object>> tags)
+    {
+        return Activity.Current?.Recorded && value > this.maxValue;
+    }
+}
+```
+
+## ExemplarReservoir
 
 Not supported.