metrics: 64bit fields should come first in structs

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

pkg/metrics/gauge.go

@@ -32,8 +32,8 @@ func RegGauge(name string, tagStrings ...string) Gauge {
 
 // GaugeSnapshot is a read-only copy of another Gauge.
 type GaugeSnapshot struct {
-	*MetricMeta
 	value int64
+	*MetricMeta
 }
 
 // Snapshot returns the snapshot.