Drop the gauge instrument (#537)

* drop gauge instrument

* Restore the benchmark and stress test for lastvalue aggregator, but remove monotonic last-value support

* Rename gauge->lastvalue and remove remaining uses of the word 'gauge'

Co-authored-by: Krzesimir Nowak <krzesimir@kinvolk.io>

example/basic/main.go

@@ -73,10 +73,16 @@ func main() {
 	tracer := global.TraceProvider().Tracer("ex.com/basic")
 	meter := global.MeterProvider().Meter("ex.com/basic")
 
-	oneMetric := meter.NewFloat64Gauge("ex.com.one",
+	commonLabels := meter.Labels(lemonsKey.Int(10), key.String("A", "1"), key.String("B", "2"), key.String("C", "3"))
+
+	oneMetricCB := func(result metric.Float64ObserverResult) {
+		result.Observe(1, commonLabels)
+	}
+	oneMetric := meter.RegisterFloat64Observer("ex.com.one", oneMetricCB,
 		metric.WithKeys(fooKey, barKey, lemonsKey),
-		metric.WithDescription("A gauge set to 1.0"),
+		metric.WithDescription("An observer set to 1.0"),
 	)
+	defer oneMetric.Unregister()
 
 	measureTwo := meter.NewFloat64Measure("ex.com.two")
 
@@ -87,11 +93,6 @@ func main() {
 		barKey.String("bar1"),
 	)
 
-	commonLabels := meter.Labels(lemonsKey.Int(10), key.String("A", "1"), key.String("B", "2"), key.String("C", "3"))
-
-	gauge := oneMetric.Bind(commonLabels)
-	defer gauge.Unbind()
-
 	measure := measureTwo.Bind(commonLabels)
 	defer measure.Unbind()
 
@@ -101,14 +102,11 @@ func main() {
 
 		trace.SpanFromContext(ctx).SetAttributes(anotherKey.String("yes"))
 
-		gauge.Set(ctx, 1)
-
 		meter.RecordBatch(
 			// Note: call-site variables added as context Entries:
 			correlation.NewContext(ctx, anotherKey.String("xyz")),
 			commonLabels,
 
-			oneMetric.Measurement(1.0),
 			measureTwo.Measurement(2.0),
 		)
 

example/prometheus/main.go

@@ -18,6 +18,7 @@ import (
 	"context"
 	"log"
 	"net/http"
+	"sync"
 	"time"
 
 	"go.opentelemetry.io/otel/api/global"
@@ -50,11 +51,21 @@ func main() {
 	defer initMeter().Stop()
 
 	meter := global.MeterProvider().Meter("ex.com/basic")
-
-	oneMetric := meter.NewFloat64Gauge("ex.com.one",
+	observerLock := new(sync.RWMutex)
+	observerValueToReport := new(float64)
+	observerLabelSetToReport := new(metric.LabelSet)
+	cb := func(result metric.Float64ObserverResult) {
+		(*observerLock).RLock()
+		value := *observerValueToReport
+		labelset := *observerLabelSetToReport
+		(*observerLock).RUnlock()
+		result.Observe(value, labelset)
+	}
+	oneMetric := meter.RegisterFloat64Observer("ex.com.one", cb,
 		metric.WithKeys(fooKey, barKey, lemonsKey),
-		metric.WithDescription("A gauge set to 1.0"),
+		metric.WithDescription("A measure set to 1.0"),
 	)
+	defer oneMetric.Unregister()
 
 	measureTwo := meter.NewFloat64Measure("ex.com.two", metric.WithKeys(key.New("A")))
 	measureThree := meter.NewFloat64Counter("ex.com.three")
@@ -64,28 +75,39 @@ func main() {
 
 	ctx := context.Background()
 
+	(*observerLock).Lock()
+	*observerValueToReport = 1.0
+	*observerLabelSetToReport = &commonLabels
+	(*observerLock).Unlock()
 	meter.RecordBatch(
 		ctx,
 		commonLabels,
-		oneMetric.Measurement(1.0),
 		measureTwo.Measurement(2.0),
 		measureThree.Measurement(12.0),
 	)
 
+	time.Sleep(5 * time.Second)
+
+	(*observerLock).Lock()
+	*observerValueToReport = 1.0
+	*observerLabelSetToReport = &notSoCommonLabels
+	(*observerLock).Unlock()
 	meter.RecordBatch(
 		ctx,
 		notSoCommonLabels,
-		oneMetric.Measurement(1.0),
 		measureTwo.Measurement(2.0),
 		measureThree.Measurement(22.0),
 	)
 
 	time.Sleep(5 * time.Second)
 
+	(*observerLock).Lock()
+	*observerValueToReport = 13.0
+	*observerLabelSetToReport = &commonLabels
+	(*observerLock).Unlock()
 	meter.RecordBatch(
 		ctx,
 		commonLabels,
-		oneMetric.Measurement(13.0),
 		measureTwo.Measurement(12.0),
 		measureThree.Measurement(13.0),
 	)