Add selector of exemplar reservoir providers to metric.Stream configuration (#5861)

Resolve https://github.com/open-telemetry/opentelemetry-go/issues/5249

### Spec

> exemplar_reservoir: A functional type that generates an exemplar
reservoir a MeterProvider will use when storing exemplars. This
functional type needs to be a factory or callback similar to aggregation
selection functionality which allows different reservoirs to be chosen
by the aggregation.

> Users can provide an exemplar_reservoir, but it is up to their
discretion. Therefore, the stream configuration parameter needs to be
structured to accept an exemplar_reservoir, but MUST NOT obligate a user
to provide one. If the user does not provide an exemplar_reservoir
value, the MeterProvider MUST apply a [default exemplar
reservoir](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exemplar-defaults).

Also,

> the reservoir MUST be given the Attributes associated with its
timeseries point either at construction so that additional sampling
performed by the reservoir has access to all attributes from a
measurement in the "offer" method.

### Changes

In sdk/metric/exemplar, add:
* `exemplar.ReservoirProvider`
* `exemplar.HistogramReservoirProvider`
* `exemplar.FixedSizeReservoirProvider`

In sdk/metric, add:
* `metric.ExemplarReservoirProviderSelector` (func Aggregation ->
ReservoirProvider)
* `metric.DefaultExemplarReservoirProviderSelector` (our default
implementation)
* `ExemplarReservoirProviderSelector` added to `metric.Stream`

Note: the only required public types are
`metric.ExemplarReservoirProviderSelector` and
`ExemplarReservoirProviderSelector` in `metric.Stream`. Others are for
convenience and readability.

### Alternatives considered

#### Add ExemplarReservoirProvider directly to metric.Stream, instead of
ExemplarReservoirProviderSelector

This would mean users can configure a `func() exemplar.Reservoir`
instead of a `func(Aggregation) func() exemplar.Reservoir`.

I don't think this complies with the statement: `This functional type
needs to be a factory or callback similar to aggregation selection
functionality which allows different reservoirs to be chosen by the
aggregation.`. I'm interpreting "allows different reservoirs to be
chosen by the aggregation" as meaning "allows different reservoirs to be
chosen **based on the** aggregation", rather than meaning that the
aggregation is somehow choosing the reservoir.

### Future work

There is some refactoring I plan to do after this to simplify the
interaction between the internal/aggregate and exemplar package. I've
omitted that from this PR to keep the diff smaller.

---------

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>
Co-authored-by: Robert Pająk <pellared@hotmail.com>

sdk/metric/example_test.go

@@ -242,6 +242,31 @@ func ExampleNewView_exponentialHistogram() {
 	)
 }
 
+func ExampleNewView_exemplarreservoirproviderselector() {
+	// Create a view that makes all metrics use a different exemplar reservoir.
+	view := metric.NewView(
+		metric.Instrument{Name: "*"},
+		metric.Stream{
+			ExemplarReservoirProviderSelector: func(agg metric.Aggregation) exemplar.ReservoirProvider {
+				// This example uses a fixed-size reservoir with a size of 10
+				// for explicit bucket histograms instead of the default
+				// bucket-aligned reservoir.
+				if _, ok := agg.(metric.AggregationExplicitBucketHistogram); ok {
+					return exemplar.FixedSizeReservoirProvider(10)
+				}
+				// Fall back to the default reservoir otherwise.
+				return metric.DefaultExemplarReservoirProviderSelector(agg)
+			},
+		},
+	)
+
+	// The created view can then be registered with the OpenTelemetry metric
+	// SDK using the WithView option.
+	_ = metric.NewMeterProvider(
+		metric.WithView(view),
+	)
+}
+
 func ExampleWithExemplarFilter_disabled() {
 	// Use exemplar.AlwaysOffFilter to disable exemplar collection.
 	_ = metric.NewMeterProvider(