Remove the sync inst from async example (#4019)

Simplify the ExampleMeter_asynchronous_multiple example test to only
include asynchronous instruments. Remove the synchronous histogram.

metric/example_test.go

@@ -79,19 +79,16 @@ func ExampleMeter_asynchronous_multiple() {
 	// This is just a sample of memory stats to record from the Memstats
 	heapAlloc, _ := meter.Int64ObservableUpDownCounter("heapAllocs")
 	gcCount, _ := meter.Int64ObservableCounter("gcCount")
-	gcPause, _ := meter.Float64Histogram("gcPause")
 
 	_, err := meter.RegisterCallback(
-		func(ctx context.Context, o metric.Observer) error {
+		func(_ context.Context, o metric.Observer) error {
 			memStats := &runtime.MemStats{}
 			// This call does work
 			runtime.ReadMemStats(memStats)
 
 			o.ObserveInt64(heapAlloc, int64(memStats.HeapAlloc))
 			o.ObserveInt64(gcCount, int64(memStats.NumGC))
 
-			// This function synchronously records the pauses
-			computeGCPauses(ctx, gcPause, memStats.PauseNs[:])
 			return nil
 		},
 		heapAlloc,
@@ -102,6 +99,3 @@ func ExampleMeter_asynchronous_multiple() {
 		panic(err)
 	}
 }
-
-// This is just an example, see the the contrib runtime instrumentation for real implementation.
-func computeGCPauses(ctx context.Context, recorder instrument.Float64Histogram, pauseBuff []uint64) {}