Unify metric API into the one otel/metric package (#4018)

* Move instrument into metric

* Update metric docs to include instrument

* Update package names

* Update all imports of sdk/metric/instrument

* Rename Option to InstrumentOption

* Deprecate otel/metric/instrument

* Add changelog entry

metric/asyncint64_test.go

@@ -0,0 +1,93 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package metric // import "go.opentelemetry.io/otel/metric"
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"go.opentelemetry.io/otel/metric/embedded"
+)
+
+func TestInt64ObservableConfiguration(t *testing.T) {
+	const (
+		token  int64 = 43
+		desc         = "Instrument description."
+		uBytes       = "By"
+	)
+
+	run := func(got int64ObservableConfig) func(*testing.T) {
+		return func(t *testing.T) {
+			assert.Equal(t, desc, got.Description(), "description")
+			assert.Equal(t, uBytes, got.Unit(), "unit")
+
+			// Functions are not comparable.
+			cBacks := got.Callbacks()
+			require.Len(t, cBacks, 1, "callbacks")
+			o := &int64Observer{}
+			err := cBacks[0](context.Background(), o)
+			require.NoError(t, err)
+			assert.Equal(t, token, o.got, "callback not set")
+		}
+	}
+
+	cback := func(ctx context.Context, obsrv Int64Observer) error {
+		obsrv.Observe(token)
+		return nil
+	}
+
+	t.Run("Int64ObservableCounter", run(
+		NewInt64ObservableCounterConfig(
+			WithDescription(desc),
+			WithUnit(uBytes),
+			WithInt64Callback(cback),
+		),
+	))
+
+	t.Run("Int64ObservableUpDownCounter", run(
+		NewInt64ObservableUpDownCounterConfig(
+			WithDescription(desc),
+			WithUnit(uBytes),
+			WithInt64Callback(cback),
+		),
+	))
+
+	t.Run("Int64ObservableGauge", run(
+		NewInt64ObservableGaugeConfig(
+			WithDescription(desc),
+			WithUnit(uBytes),
+			WithInt64Callback(cback),
+		),
+	))
+}
+
+type int64ObservableConfig interface {
+	Description() string
+	Unit() string
+	Callbacks() []Int64Callback
+}
+
+type int64Observer struct {
+	embedded.Int64Observer
+	Observable
+	got int64
+}
+
+func (o *int64Observer) Observe(v int64, _ ...ObserveOption) {
+	o.got = v
+}