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

doc.go

@@ -69,5 +69,51 @@ used as a default.
 		defer span.End()
 		// ...
 	}
+
+Metric Measurements
+
+Measurements can be made about an operation being performed or the state of a
+system in general. These measurements can be crucial to the reliable operation
+of code and provide valuable insights about the inner workings of a system.
+
+Measurements are made using instruments provided by this package. The type of
+instrument used will depend on the type of measurement being made and of what
+part of a system is being measured.
+
+Instruments are categorized as Synchronous or Asynchronous and independently
+as Adding or Grouping. Synchronous instruments are called by the user with a
+Context. Asynchronous instruments are called by the SDK during collection.
+Additive instruments are semantically intended for capturing a sum. Grouping
+instruments are intended for capturing a distribution.
+
+Additive instruments may be monotonic, in which case they are non-decreasing
+and naturally define a rate.
+
+The synchronous instrument names are:
+
+  Counter:           additive, monotonic
+  UpDownCounter:     additive
+  ValueRecorder:     grouping
+
+and the asynchronous instruments are:
+
+  SumObserver:       additive, monotonic
+  UpDownSumObserver: additive
+  ValueObserver:     grouping
+
+All instruments are provided with support for either float64 or int64 input
+values.
+
+An instrument is created using a Meter. Additionally, a Meter is used to
+record batches of synchronous measurements or asynchronous observations. A
+Meter is obtained using a MeterProvider. A Meter, like a Tracer, is unique to
+the instrumentation it instruments and must be named and versioned when
+created with a MeterProvider with the name and version of the instrumentation
+library.
+
+Instrumentation should be designed to accept a MeterProvider from which it can
+create its own unique Meter. Alternatively, the registered global
+MeterProvider from the go.opentelemetry.io/otel/api/global package can be used
+as a default.
 */
 package otel // import "go.opentelemetry.io/otel"