Fixes issue #8199

Benchmarks

```
benchmark              old ns/op     new ns/op     delta
BenchmarkMarshal-8     1216          1007          -17.19%

benchmark              old allocs     new allocs     delta
BenchmarkMarshal-8     4              2              -50.00%

benchmark              old bytes     new bytes     delta
BenchmarkMarshal-8     416           256           -38.46%
```

models/points.go

@@ -1773,40 +1773,35 @@ func (a Tags) HashKey() []byte {
 		return nil
 	}
 
+	// Type invariant: Tags are sorted
+
 	escaped := make(Tags, 0, len(a))
+	sz := 0
 	for _, t := range a {
 		ek := escapeTag(t.Key)
 		ev := escapeTag(t.Value)
 
 		if len(ev) > 0 {
 			escaped = append(escaped, Tag{Key: ek, Value: ev})
+			sz += len(ek) + len(ev)
 		}
 	}
 
-	// Extract keys and determine final size.
-	sz := len(escaped) + (len(escaped) * 2) // separators
-	keys := make([][]byte, len(escaped)+1)
-	for i, t := range escaped {
-		keys[i] = t.Key
-		sz += len(t.Key) + len(t.Value)
-	}
-	keys = keys[:len(escaped)]
-	sort.Sort(byteSlices(keys))
+	sz += len(escaped) + (len(escaped) * 2) // separators
 
 	// Generate marshaled bytes.
 	b := make([]byte, sz)
 	buf := b
 	idx := 0
-	for i, k := range keys {
+	for _, k := range escaped {
 		buf[idx] = ','
 		idx++
-		copy(buf[idx:idx+len(k)], k)
-		idx += len(k)
+		copy(buf[idx:idx+len(k.Key)], k.Key)
+		idx += len(k.Key)
 		buf[idx] = '='
 		idx++
-		v := escaped[i].Value
-		copy(buf[idx:idx+len(v)], v)
-		idx += len(v)
+		copy(buf[idx:idx+len(k.Value)], k.Value)
+		idx += len(k.Value)
 	}
 	return b[:idx]
 }

models/points_test.go

@@ -40,6 +40,16 @@ func TestMarshal(t *testing.T) {
 	}
 }
 
+func TestTags_HashKey(t *testing.T) {
+	tags = models.NewTags(map[string]string{"A FOO": "bar", "APPLE": "orange", "host": "serverA", "region": "uswest"})
+	got := tags.HashKey()
+	if exp := ",A\\ FOO=bar,APPLE=orange,host=serverA,region=uswest"; string(got) != exp {
+		t.Log("got: ", string(got))
+		t.Log("exp: ", exp)
+		t.Error("invalid match")
+	}
+}
+
 func BenchmarkMarshal(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		tags.HashKey()