otlpmetric: StartTimeUnixNano for Gauge metric points is always set to "11651379494838206464"

[pellared]

Repro steps

Code:

package main

import (
	"context"

	"go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetrichttp"
	"go.opentelemetry.io/otel/metric"
	metricsdk "go.opentelemetry.io/otel/sdk/metric"
)

func main() {
	ctx := context.Background()
	exp, err := otlpmetrichttp.New(ctx, otlpmetrichttp.WithInsecure())
	if err != nil {
		panic(err)
	}

	meterProvider := metricsdk.NewMeterProvider(metricsdk.WithReader(metricsdk.NewPeriodicReader(exp)))
	defer func() {
		if err := meterProvider.Shutdown(ctx); err != nil {
			panic(err)
		}
	}()

	meter := meterProvider.Meter("go-playground")

	_, err = meter.Int64ObservableGauge("async-gauge", metric.WithInt64Callback(
		func(ctx context.Context, io metric.Int64Observer) error {
			io.Observe(123)
			return nil
		},
	))
	if err != nil {
		panic(err)
	}
}

Add a breakpoint here:

opentelemetry-go/exporters/otlp/otlpmetric/internal/exporter.go

Lines 54 to 56 in 8e25817

	otlpRm, err := transform.ResourceMetrics(rm)
	// Best effort upload of transformable metrics.
	e.clientMu.Lock()

and run the debugger.

Actual

You will notice that the input (rm *metricdata.ResourceMetrics) contains z zero value of time.Time for StartTime and after serializing to proto the payload contains StartTimeUnixNano = 11651379494838206464 which is far from being correct: Tue 21 March 2339 22:18:14.838 UTC

Expected

1. Fix the transform.ResourceMetrics(rm) to properly handle time.Time zero values. Take notice: https://pkg.go.dev/time#Time.UnixNano. Probably we could set StartTimeUnixNano = 0.
2. (nice to have, this might be tracked as a separate issue) The initial StartTime could be set as suggested per specification: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/data-model.md#gauge. EDIT: I created sdk/metric: Set StartTime for gauge metric points #4269

Discussed in #4265

@anil181 thank you for reporting the issue.