[WIP] factor out trace functionality into separate package and add Go docs

daemon/snapper/snapper.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/pkg/errors"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/filters"
@@ -133,7 +134,7 @@ func PeriodicFromConfig(g *config.Global, fsf *filters.DatasetMapFilter, in *con
 }
 
 func (s *Snapper) Run(ctx context.Context, snapshotsTaken chan<- struct{}) {
-	defer logging.WithSpanFromStackUpdateCtx(&ctx)()
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 	getLogger(ctx).Debug("start")
 	defer getLogger(ctx).Debug("stop")