fix 64-bit aligment for 32-bit platforms

sync/atomic must be used with 64-bit aligned fields, and that alignment is difficult to
ensure unless the field is the first one in the struct.

https://golang.org/pkg/sync/atomic/#pkg-note-BUG.

.changelog/10515.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+agent: fix a panic on 32-bit platforms caused by misaligned struct fields used with sync/atomic.
+```

agent/grpc/stats.go

@@ -52,8 +52,11 @@ var defaultMetrics = metrics.Default
 // statsHandler is a grpc/stats.StatsHandler which emits connection and
 // request metrics to go-metrics.
 type statsHandler struct {
+	// activeConns is used with sync/atomic and MUST be 64-bit aligned. To ensure
+	// alignment on 32-bit platforms this field must remain the first field in
+	// the struct. See https://golang.org/pkg/sync/atomic/#pkg-note-BUG.
+	activeConns uint64
 	metrics     *metrics.Metrics
-	activeConns uint64 // must be 8-byte aligned for atomic access
 }
 
 func newStatsHandler(m *metrics.Metrics) *statsHandler {
@@ -103,10 +106,11 @@ func (c *statsHandler) HandleConn(_ context.Context, s stats.ConnStats) {
 }
 
 type activeStreamCounter struct {
-	metrics *metrics.Metrics
-	// count of the number of open streaming RPCs on a server. It is accessed
-	// atomically.
+	// count is used with sync/atomic and MUST be 64-bit aligned. To ensure
+	// alignment on 32-bit platforms this field must remain the first field in
+	// the struct. See https://golang.org/pkg/sync/atomic/#pkg-note-BUG.
 	count   uint64
+	metrics *metrics.Metrics
 }
 
 // GRPCCountingStreamInterceptor is a grpc.ServerStreamInterceptor that emits a

tlsutil/config.go

@@ -175,6 +175,11 @@ type manual struct {
 // Configurator receives an initial TLS configuration from agent configuration,
 // and receives updates from config reloads, auto-encrypt, and auto-config.
 type Configurator struct {
+	// version is increased each time the Configurator is updated. Must be accessed
+	// using sync/atomic. Also MUST be the first field in this struct to ensure
+	// 64-bit alignment. See https://golang.org/pkg/sync/atomic/#pkg-note-BUG.
+	version uint64
+
 	// lock synchronizes access to all fields on this struct except for logger and version.
 	lock    sync.RWMutex
 	base    *Config
@@ -188,9 +193,6 @@ type Configurator struct {
 	// logger is not protected by a lock. It must never be changed after
 	// Configurator is created.
 	logger hclog.Logger
-	// version is increased each time the Configurator is updated. Must be accessed
-	// using sync/atomic.
-	version uint64
 }
 
 // NewConfigurator creates a new Configurator and sets the provided