fix(models): reset provided slice correctly (#23223)

A preallocated slice needs to be cleared to be used with append,
otherwise the existing elements will be seen in the result and this does
not appear to be the intention. The bug doesn't seem to have caused
issues as no callsites use a preallocated slice.

models/points.go

@@ -2119,7 +2119,7 @@ func (a Tags) KeyValues(v [][]byte) [][]byte {
 	if cap(v) < l {
 		v = make([][]byte, 0, l)
 	} else {
-		v = v[:l]
+		v = v[:0]
 	}
 	for i := range a {
 		v = append(v, a[i].Key, a[i].Value)

models/points_test.go

@@ -13,6 +13,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/google/go-cmp/cmp"
 	"github.com/influxdata/influxdb/v2/models"
 )
 
@@ -2428,6 +2429,27 @@ func TestParseName(t *testing.T) {
 	}
 }
 
+func TestTags_KeyValues(t *testing.T) {
+	tags := models.NewTags(map[string]string{
+		"tag0": "v0",
+		"tag1": "v1",
+		"tag2": "v2",
+	})
+
+	got := tags.KeyValues(nil)
+	exp := [][]byte{[]byte("tag0"), []byte("v0"), []byte("tag1"), []byte("v1"), []byte("tag2"), []byte("v2")}
+	if !cmp.Equal(got, exp) {
+		t.Errorf("unexpected, -got/+exp\n%s", cmp.Diff(got, exp))
+	}
+
+	v := make([][]byte, 0, 10)
+	v = tags.KeyValues(v)
+	got2 := tags.KeyValues(v)
+	if !cmp.Equal(got2, exp) {
+		t.Errorf("unexpected, -got/+exp\n%s", cmp.Diff(got2, exp))
+	}
+}
+
 func BenchmarkEscapeStringField_Plain(b *testing.B) {
 	s := "nothing special"
 	for i := 0; i < b.N; i++ {