SumObserver sums up observations

[riptl]

otel version: v0.14.0

Expected behavior

The OpenTelemetry specification claims:

Asynchronous, adding instruments (e.g., SumObserver) are used to capture sums directly. This means, for example, that in any sequence of SumObserver observations for a given instrument and label set, the Last Value defines the sum of the instrument.

The comment on Int64SumObserver claims:

// Int64SumObserver is a metric that captures a precomputed sum of
// int64 values at a point in time.

I expect that calling Int64SumObserver.Observe(v) only returns the latest observation instead of a sum of all observations.

Actual Behavior

Int64SumObserver sums up all observations instead of using the last value.

This makes it impossible to define Prometheus COUNTERs from external sources.
I found this bug when trying to export a counter in Redis.

Code Example

package example

import (
	"context"
	"net/http"
	"net/http/httptest"
	"net/url"
	"testing"
	"time"

	"github.com/stretchr/testify/require"
	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/exporters/metric/prometheus"
	"go.opentelemetry.io/otel/metric"
)

func TestSums(t *testing.T) {
	// Install new Prometheus exporter
	exporter, err := prometheus.InstallNewPipeline(prometheus.Config{})
	require.NoError(t, err)
	// Define a sum observer that always sees "100".
	_, err = otel.Meter("test").NewInt64SumObserver("test_sum", func(ctx context.Context, result metric.Int64ObserverResult) {
		result.Observe(100)
	})
	require.NoError(t, err)
	// Send some requests
	req := &http.Request{
		Method: "GET",
		URL: &url.URL{
			Scheme: "http",
			Host:   "localhost",
			Path:   "/metrics",
		},
	}
	exporter.ServeHTTP(httptest.NewRecorder(), req)
	time.Sleep(15*time.Second)
	exporter.ServeHTTP(httptest.NewRecorder(), req)
	time.Sleep(15*time.Second)
	exporter.ServeHTTP(httptest.NewRecorder(), req)
	time.Sleep(15*time.Second)
	// Print last request
	recorder := httptest.NewRecorder()
	exporter.ServeHTTP(recorder, req)
	body := recorder.Body.String()
	t.Log("Exported Metrics\n" + body)
}

Outputs

# HELP test_sum 
# TYPE test_sum counter
test_sum 400

[jmacd]

#1381