Move metric api package into otel (#1252)

* Update metric Kind to InstrumentKind

* Update all the other modules with Kind rename

* Update metric Descriptor with instrument Kind rename

* Update other modules with Descriptor method rename

* Update OTLP exporter test field name

* Rename kind filenames

* Add changes to CHANGELOG

* Fix documentation for Grouping and PrecomputedSum

* Rename meter.go to metric.go

* Move descriptor.go into metric.go

* Move must.go into metric.go

* Move instruments into metric_instrument.go

* Rename metric api_test.go to metric_test.go

* Move instrumentkind_test.go into metric_test.go

* Rename sdkapi.go metric_sdkapi.go

* Move api/metric into otel

* Update to use moved packages

* Rename otel.go to error_handler.go

* Add changes to CHANGELOG

* Fix merge conflict resolution error

error_handler.go

@@ -0,0 +1,22 @@
+// 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 otel
+
+// ErrorHandler handles irremediable events.
+type ErrorHandler interface {
+	// Handle handles any error deemed irremediable by an OpenTelemetry
+	// component.
+	Handle(error)
+}