Add comments and test for 64-bit field alignment (#418)

* Add comments on needed filed alignment

Add comment about alignment requirements to all struct fields who's
values are passed to 64-bit atomic operations.

Update any struct's field ordering if one or more of those fields has
alignment requirements to support 64-bit atomic operations.

* Add 64-bit alignment tests

Most `struct` that have field alignment requirements are now statically
validated prior to testing. The only `struct`s not validated that have
these requirements are ones defined in tests themselves where multiple
`TestMain` functions would be needed to test them. Given the fields are
already identified with comments specifying the alignment requirements
and they are in the test themselves, this seems like an OK omission.

Co-authored-by: Liz Fong-Jones <elizabeth@ctyalcove.org>

sdk/metric/aggregator/array/array_test.go

@@ -18,16 +18,34 @@ import (
 	"context"
 	"fmt"
 	"math"
+	"os"
 	"testing"
+	"unsafe"
 
 	"github.com/stretchr/testify/require"
 
 	"go.opentelemetry.io/otel/api/core"
+	ottest "go.opentelemetry.io/otel/internal/testing"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
 	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/test"
 )
 
+// Ensure struct alignment prior to running tests.
+func TestMain(m *testing.M) {
+	fields := []ottest.FieldOffset{
+		{
+			Name:   "Aggregator.ckptSum",
+			Offset: unsafe.Offsetof(Aggregator{}.ckptSum),
+		},
+	}
+	if !ottest.Aligned8Byte(fields, os.Stderr) {
+		os.Exit(1)
+	}
+
+	os.Exit(m.Run())
+}
+
 type updateTest struct {
 	count    int
 	absolute bool