Precompute encodeWithKeys buffer size to avoid resizes (#1269)

This is strictly an optimization. Right now we always undersize the buffer then later increase it.
This changes the code to accurately size it the first time to ensure we never re-allocate.

```
name                old time/op    new time/op    delta
RecordReqCommand-6    2.26µs ± 5%    2.10µs ± 5%   -7.39%  (p=0.000 n=10+10)
RecordViaStats-6      2.70µs ± 5%    2.53µs ± 4%   -6.31%  (p=0.000 n=10+10)

name                old alloc/op   new alloc/op   delta
RecordReqCommand-6      426B ± 0%      384B ± 0%   -9.86%  (p=0.000 n=10+10)
RecordViaStats-6        594B ± 0%      552B ± 0%   -7.07%  (p=0.000 n=10+10)

name                old allocs/op  new allocs/op  delta
RecordReqCommand-6      25.0 ± 0%      17.0 ± 0%  -32.00%  (p=0.000 n=10+10)
RecordViaStats-6        28.0 ± 0%      20.0 ± 0%  -28.57%  (p=0.000 n=10+10)
```

stats/view/collector.go

@@ -59,8 +59,15 @@ func (c *collector) clearRows() {
 // encodeWithKeys encodes the map by using values
 // only associated with the keys provided.
 func encodeWithKeys(m *tag.Map, keys []tag.Key) []byte {
+	// Compute the buffer length we will need ahead of time to avoid resizing later
+	reqLen := 0
+	for _, k := range keys {
+		s, _ := m.Value(k)
+		// We will store each key + its length
+		reqLen += len(s) + 1
+	}
 	vb := &tagencoding.Values{
-		Buffer: make([]byte, len(keys)),
+		Buffer: make([]byte, reqLen),
 	}
 	for _, k := range keys {
 		v, _ := m.Value(k)