Write performance optimization (2.5x) (#428)

Disable performance metrics (~15% gain)
Perform sort only if metrics were out of order(100% gain)

Author: pavius

Diff for: internal/pkg/performance/metrics.go

@@ -190,8 +190,5 @@ func (mr *MetricReporter) isEnabled() bool {
 }
 
 func (mr *MetricReporter) isRunning() bool {
-	mr.lock.Lock()
-	defer mr.lock.Unlock()
-
-	return mr.running
+	return false
 }

Diff for: pkg/appender/store.go

@@ -232,7 +232,7 @@ func (cs *chunkStore) Append(t int64, v interface{}) {
 
 	cs.pending = append(cs.pending, pendingData{t: t, v: v})
 	// If the new time is older than previous times, sort the list
-	if len(cs.pending) > 1 && cs.pending[len(cs.pending)-2].t < t {
+	if len(cs.pending) > 1 && cs.pending[len(cs.pending)-2].t > t {
 		sort.Sort(cs.pending)
 	}
 }