Golang opentelemetry-go draft API (incomplete) (#9)

* Work in progress from https://github.com/lightstep/opentelemetry-golang-prototype

* Renames

* Rename

* Finish rename

* Rename packages

* README

api/metric/api.go

@@ -0,0 +1,60 @@
+package metric
+
+import (
+	"github.com/open-telemetry/opentelemetry-go/api/core"
+	"github.com/open-telemetry/opentelemetry-go/api/tag"
+	"github.com/open-telemetry/opentelemetry-go/api/unit"
+)
+
+type (
+	Metric interface {
+		Measure() core.Measure
+
+		DefinitionID() core.EventID
+
+		Type() MetricType
+		Fields() []core.Key
+		Err() error
+
+		base() *baseMetric
+	}
+
+	MetricType int
+)
+
+const (
+	Invalid MetricType = iota
+	GaugeInt64
+	GaugeFloat64
+	DerivedGaugeInt64
+	DerivedGaugeFloat64
+	CumulativeInt64
+	CumulativeFloat64
+	DerivedCumulativeInt64
+	DerivedCumulativeFloat64
+)
+
+type (
+	Option func(*baseMetric, *[]tag.Option)
+)
+
+// WithDescription applies provided description.
+func WithDescription(desc string) Option {
+	return func(_ *baseMetric, to *[]tag.Option) {
+		*to = append(*to, tag.WithDescription(desc))
+	}
+}
+
+// WithUnit applies provided unit.
+func WithUnit(unit unit.Unit) Option {
+	return func(_ *baseMetric, to *[]tag.Option) {
+		*to = append(*to, tag.WithUnit(unit))
+	}
+}
+
+// WithKeys applies the provided dimension keys.
+func WithKeys(keys ...core.Key) Option {
+	return func(bm *baseMetric, _ *[]tag.Option) {
+		bm.keys = keys
+	}
+}