ForceFlush and Shutdown do not collect pending measurements

[MrAlias]

Both the MeterProvider's ForceFlush and Shutdown methods say they flush pending telemetry:

opentelemetry-go/sdk/metric/provider.go

Line 90 in 038248b

// ForceFlush flushes all pending telemetry.

opentelemetry-go/sdk/metric/provider.go

Lines 105 to 106 in 038248b

	// Shutdown shuts down the MeterProvider flushing all pending telemetry and
	// releasing any held computational resources.

They currently do not do this for PeriodicReaders.

Example

main.go:

package main

import (
	"context"
	"log"

	"go.opentelemetry.io/otel/exporters/stdout/stdoutmetric"
	"go.opentelemetry.io/otel/sdk/metric"
)

func main() {
	ctx := context.Background()

	exporter, err := stdoutmetric.New()
	if err != nil {
		log.Fatalln("failed to create stdout exporter:", err)
	}

	meterProvider := metric.NewMeterProvider(
		metric.WithReader(metric.NewPeriodicReader(exporter)),
	)
	meter := meterProvider.Meter("my-meter")

	counter, err := meter.SyncFloat64().Counter("my-counter")
	if err != nil {
		log.Fatalln("failed to initialize instrument:", err)
	}

	counter.Add(ctx, 12.0)
	log.Println("counted", 12.0)

	err = meterProvider.Shutdown(ctx)
	if err != nil {
		log.Fatalln("failed to shutdown:", err)
	}
	log.Println("shutdown")
}

go.mod:

go 1.19

require (
	go.opentelemetry.io/otel/exporters/stdout/stdoutmetric v0.32.0
	go.opentelemetry.io/otel/sdk/metric v0.32.0
)

require (
	github.com/go-logr/logr v1.2.3 // indirect
	github.com/go-logr/stdr v1.2.2 // indirect
	go.opentelemetry.io/otel v1.10.0 // indirect
	go.opentelemetry.io/otel/metric v0.32.0 // indirect
	go.opentelemetry.io/otel/sdk v1.10.0 // indirect
	go.opentelemetry.io/otel/trace v1.10.0 // indirect
	golang.org/x/sys v0.0.0-20210510120138-977fb7262007 // indirect
)

This outputs:

$ go run .
2022/09/20 16:50:56 counted 12
2022/09/20 16:50:56 shutdown

What I would expect to see:

go run .
2022/09/20 16:52:13 counted 12
{
	"Resource": null,
	"ScopeMetrics": [
		{
			"Scope": {
				"Name": "my-meter",
				"Version": "",
				"SchemaURL": ""
			},
			"Metrics": [
				{
					"Name": "my-counter",
					"Description": "",
					"Unit": "",
					"Data": {
						"DataPoints": [
							{
								"Attributes": [],
								"StartTime": "2022-09-20T16:52:13.742478572-07:00",
								"Time": "2022-09-20T16:52:14.742870752-07:00",
								"Value": 12
							}
						],
						"Temporality": "CumulativeTemporality",
						"IsMonotonic": true
					}
				}
			]
		}
	]
}
2022/09/20 16:52:15 shutdown

[MrAlias]

Verified the changes proposed in #3220 resolve this issue based on the example provided in the description.