func (f *EntryAssertFormatter) Format(entry *logrus.Entry) ([]byte, error) {
	f.appendThreadSafe(*entry)
	return f.Underlying.Format(entry)
}
Format appends each entry to entries slice
package spy
import (
	"sync"
	"github.com/sirupsen/logrus"
)
type EntryAssertFormatter struct {
	entries    []logrus.Entry
	Underlying logrus.Formatter
	mu         sync.RWMutex
}