Reduce allocations for queries on `HEAD` (#417)

Some benchmarks for HEAD and allocate the correct slice size in LabelValues , we already know what it'll be

This is ~15% time improvement, and ~25% allocation improvement:


```
benchmark                             old ns/op     new ns/op     delta
BenchmarkHeadPostingForMatchers-4     74452         63514         -14.69%

benchmark                             old allocs     new allocs     delta
BenchmarkHeadPostingForMatchers-4     20             13             -35.00%

benchmark                             old bytes     new bytes     delta
BenchmarkHeadPostingForMatchers-4     5425          3137          -42.18%
```

Signed-off-by: Thomas Jackson <jacksontj.89@gmail.com>

head.go

@@ -1014,14 +1014,13 @@ func (h *headIndexReader) LabelValues(names ...string) (index.StringTuples, erro
 	if len(names) != 1 {
 		return nil, errInvalidSize
 	}
-	var sl []string
 
 	h.head.symMtx.RLock()
-	defer h.head.symMtx.RUnlock()
-
+	sl := make([]string, 0, len(h.head.values[names[0]]))
 	for s := range h.head.values[names[0]] {
 		sl = append(sl, s)
 	}
+	h.head.symMtx.RUnlock()
 	sort.Strings(sl)
 
 	return index.NewStringTuples(sl, len(names))

head_bench_test.go

@@ -0,0 +1,59 @@
+package tsdb
+
+import (
+	"strconv"
+	"sync/atomic"
+	"testing"
+
+	"github.com/prometheus/tsdb/labels"
+	"github.com/prometheus/tsdb/testutil"
+)
+
+func BenchmarkHeadStripeSeriesCreate(b *testing.B) {
+	// Put a series, select it. GC it and then access it.
+	h, err := NewHead(nil, nil, nil, 1000)
+	testutil.Ok(b, err)
+	defer h.Close()
+
+	for i := 0; i < b.N; i++ {
+		h.getOrCreate(uint64(i), labels.FromStrings("a", strconv.Itoa(i)))
+	}
+}
+
+func BenchmarkHeadStripeSeriesCreateParallel(b *testing.B) {
+	// Put a series, select it. GC it and then access it.
+	h, err := NewHead(nil, nil, nil, 1000)
+	testutil.Ok(b, err)
+	defer h.Close()
+
+	var count int64
+
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			i := atomic.AddInt64(&count, 1)
+			h.getOrCreate(uint64(i), labels.FromStrings("a", strconv.Itoa(int(i))))
+		}
+	})
+}
+
+// TODO: generalize benchmark and pass all postings for matchers here
+func BenchmarkHeadPostingForMatchers(b *testing.B) {
+	// Put a series, select it. GC it and then access it.
+	h, err := NewHead(nil, nil, nil, 1000)
+	testutil.Ok(b, err)
+	defer h.Close()
+
+	// TODO: vary number of series
+	for i := 0; i < 100; i++ {
+		h.getOrCreate(uint64(i), labels.FromStrings("a", strconv.Itoa(i)))
+	}
+
+	b.ResetTimer()
+
+	all, _ := labels.NewRegexpMatcher("a", ".*")
+
+	for i := 0; i < b.N; i++ {
+		_, err := PostingsForMatchers(h.indexRange(0, 1000), all)
+		testutil.Ok(b, err)
+	}
+}