Testing: Run sync measure benchmarks in parallel (#7113)

I am looking into
https://promlabs.com/blog/2025/07/17/why-i-recommend-native-prometheus-instrumentation-over-opentelemetry/#comparing-counter-increment-performance,
which seems to suggest the OTel metrics SDK performs poorly when a
counter is incremented concurrently. It is potentially a bit of an
artificial benchmark, but does suggest there is some contention beyond
just the fact that they are incrementing an atomic integer...

Original benchmarks from the blog post:
https://github.com/promlabs/prometheus-otel-benchmarks/blob/main/otel_test.go

```
$ go test -run=xxxxxMatchNothingxxxxx -cpu=24 -test.benchtime=1s -bench=BenchmarkSyncMeasure/NoView/
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/sdk/metric
cpu: Intel(R) Xeon(R) CPU @ 2.20GHz
BenchmarkSyncMeasure/NoView/Int64Counter/Attributes/0-24         	 3946789	       313.2 ns/op
BenchmarkSyncMeasure/NoView/Int64Counter/Attributes/1-24         	 3420992	       374.4 ns/op
BenchmarkSyncMeasure/NoView/Int64Counter/Attributes/10-24        	  574608	      1745 ns/op
BenchmarkSyncMeasure/NoView/Float64Counter/Attributes/0-24       	 3996166	       281.1 ns/op
BenchmarkSyncMeasure/NoView/Float64Counter/Attributes/1-24       	 3091573	       367.1 ns/op
BenchmarkSyncMeasure/NoView/Float64Counter/Attributes/10-24      	  705693	      1660 ns/op
BenchmarkSyncMeasure/NoView/Int64UpDownCounter/Attributes/0-24   	 4098727	       296.4 ns/op
BenchmarkSyncMeasure/NoView/Int64UpDownCounter/Attributes/1-24   	 3029276	       355.4 ns/op
BenchmarkSyncMeasure/NoView/Int64UpDownCounter/Attributes/10-24  	  605174	      1803 ns/op
BenchmarkSyncMeasure/NoView/Float64UpDownCounter/Attributes/0-24 	 4057765	       298.6 ns/op
BenchmarkSyncMeasure/NoView/Float64UpDownCounter/Attributes/1-24 	 3384812	       366.9 ns/op
BenchmarkSyncMeasure/NoView/Float64UpDownCounter/Attributes/10-24         	  714900	      1742 ns/op
BenchmarkSyncMeasure/NoView/Int64Histogram/Attributes/0-24                	 3274644	       364.3 ns/op
BenchmarkSyncMeasure/NoView/Int64Histogram/Attributes/1-24                	 3780115	       316.1 ns/op
BenchmarkSyncMeasure/NoView/Int64Histogram/Attributes/10-24               	 1294364	       993.5 ns/op
BenchmarkSyncMeasure/NoView/Float64Histogram/Attributes/0-24              	 3543817	       343.2 ns/op
BenchmarkSyncMeasure/NoView/Float64Histogram/Attributes/1-24              	 3523102	       335.8 ns/op
BenchmarkSyncMeasure/NoView/Float64Histogram/Attributes/10-24             	 1329352	       956.3 ns/op
PASS
ok  	go.opentelemetry.io/otel/sdk/metric	27.504s
```
```
$ go test -run=xxxxxMatchNothingxxxxx -cpu=1 -test.benchtime=1s -bench=BenchmarkSyncMeasure/NoView/
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/sdk/metric
cpu: Intel(R) Xeon(R) CPU @ 2.20GHz
BenchmarkSyncMeasure/NoView/Int64Counter/Attributes/0         	 9905773	       121.3 ns/op
BenchmarkSyncMeasure/NoView/Int64Counter/Attributes/1         	 4079145	       296.5 ns/op
BenchmarkSyncMeasure/NoView/Int64Counter/Attributes/10        	  781627	      1531 ns/op
BenchmarkSyncMeasure/NoView/Float64Counter/Attributes/0       	10017988	       120.2 ns/op
BenchmarkSyncMeasure/NoView/Float64Counter/Attributes/1       	 4055418	       296.4 ns/op
BenchmarkSyncMeasure/NoView/Float64Counter/Attributes/10      	  761139	      1540 ns/op
BenchmarkSyncMeasure/NoView/Int64UpDownCounter/Attributes/0   	10017126	       121.1 ns/op
BenchmarkSyncMeasure/NoView/Int64UpDownCounter/Attributes/1   	 4037232	       295.3 ns/op
BenchmarkSyncMeasure/NoView/Int64UpDownCounter/Attributes/10  	  757010	      1539 ns/op
BenchmarkSyncMeasure/NoView/Float64UpDownCounter/Attributes/0 	10122925	       119.0 ns/op
BenchmarkSyncMeasure/NoView/Float64UpDownCounter/Attributes/1 	 4070942	       293.8 ns/op
BenchmarkSyncMeasure/NoView/Float64UpDownCounter/Attributes/10         	  788176	      1542 ns/op
BenchmarkSyncMeasure/NoView/Int64Histogram/Attributes/0                	10794142	       110.8 ns/op
BenchmarkSyncMeasure/NoView/Int64Histogram/Attributes/1                	 5929494	       201.0 ns/op
BenchmarkSyncMeasure/NoView/Int64Histogram/Attributes/10               	 1449292	       825.4 ns/op
BenchmarkSyncMeasure/NoView/Float64Histogram/Attributes/0              	10875385	       110.1 ns/op
BenchmarkSyncMeasure/NoView/Float64Histogram/Attributes/1              	 5903116	       202.4 ns/op
BenchmarkSyncMeasure/NoView/Float64Histogram/Attributes/10             	 1459578	       827.4 ns/op
PASS
ok  	go.opentelemetry.io/otel/sdk/metric	25.688s
```

Results are significantly worse (almost > 2x in some cases) with
parallelism, but don't initially seem as bad as the blog post suggests.
I only have 24 cores, so I can't test higher numbers.

Do we want to have parallel benchmarks in addition to our current
non-parallel ones?

Author: dashpole

sdk/metric/benchmark_test.go

@@ -113,19 +113,19 @@ func benchMeasAttrs(meas measF) func(*testing.B) {
 	return func(b *testing.B) {
 		b.Run("Attributes/0", func(b *testing.B) {
 			f := meas(*attribute.EmptySet())
-			b.ReportAllocs()
-			b.ResetTimer()
-			for n := 0; n < b.N; n++ {
-				f()
-			}
+			b.RunParallel(func(pb *testing.PB) {
+				for pb.Next() {
+					f()
+				}
+			})
 		})
 		b.Run("Attributes/1", func(b *testing.B) {
 			f := meas(attribute.NewSet(attribute.Bool("K", true)))
-			b.ReportAllocs()
-			b.ResetTimer()
-			for n := 0; n < b.N; n++ {
-				f()
-			}
+			b.RunParallel(func(pb *testing.PB) {
+				for pb.Next() {
+					f()
+				}
+			})
 		})
 		b.Run("Attributes/10", func(b *testing.B) {
 			n := 10
@@ -135,11 +135,11 @@ func benchMeasAttrs(meas measF) func(*testing.B) {
 				attrs = append(attrs, attribute.Int(strconv.Itoa(i), i))
 			}
 			f := meas(attribute.NewSet(attrs...))
-			b.ReportAllocs()
-			b.ResetTimer()
-			for n := 0; n < b.N; n++ {
-				f()
-			}
+			b.RunParallel(func(pb *testing.PB) {
+				for pb.Next() {
+					f()
+				}
+			})
 		})
 	}
 }