Move exemplar types to non-internal package (#5747)

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

This makes all existing types designed to implement the public Exemplar
API public by moving most of `internal/exemplar` to `exemplar`. The only
types that are not being made public are `exemplar.Drop`, and
`exemplar.FilteredReservoir`. Those types are moved to
`internal/aggregate`, and are renamed to `DropReservoir` and
`FilteredExemplarReservoir`.

The following types are made public:

* `exemplar.Exemplar`
* `exemplar.Filter`
* `exemplar.SampledFilter`
* `exemplar.AlwaysOnFilter`
* `exemplar.HistogramReservoir`
* `exemplar.FixedSizeReservoir`
* `exemplar.Reservoir`
* `exemplar.Value`
* `exemplar.ValueType`

sdk/metric/internal/aggregate/drop_test.go

@@ -0,0 +1,26 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package aggregate
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"go.opentelemetry.io/otel/sdk/metric/exemplar"
+)
+
+func TestDrop(t *testing.T) {
+	t.Run("Int64", testDropFiltered[int64])
+	t.Run("Float64", testDropFiltered[float64])
+}
+
+func testDropFiltered[N int64 | float64](t *testing.T) {
+	r := DropReservoir[N]()
+
+	var dest []exemplar.Exemplar
+	r.Collect(&dest)
+
+	assert.Empty(t, dest, "non-sampled context should not be offered")
+}