Fix data races with shared global metrics

I don't think this would have been a problem with real-world usage of k6 (for now), but the way metrics are defined should probably be refactored to avoid shared global variables.

core/engine.go

@@ -57,7 +57,7 @@ type Engine struct {
 	logger *log.Logger
 
 	Metrics     map[string]*stats.Metric
-	MetricsLock sync.RWMutex
+	MetricsLock sync.Mutex
 
 	// Assigned to metrics upon first received sample.
 	thresholds map[string]stats.Thresholds
@@ -319,7 +319,7 @@ func (e *Engine) processSamples(samples ...stats.Sample) {
 	for i, sample := range samples {
 		m, ok := e.Metrics[sample.Metric.Name]
 		if !ok {
-			m = sample.Metric
+			m = stats.New(sample.Metric.Name, sample.Metric.Type, sample.Metric.Contains)
 			m.Thresholds = e.thresholds[m.Name]
 			m.Submetrics = e.submetrics[m.Name]
 			e.Metrics[m.Name] = m

core/engine_test.go

@@ -548,7 +548,7 @@ func TestSentReceivedMetrics(t *testing.T) {
 	getTestCase := func(t *testing.T, tc testCase) func(t *testing.T) {
 		return func(t *testing.T) {
 			//TODO: figure out why it fails if we uncomment this:
-			//t.Parallel()
+			t.Parallel()
 			r, err := js.New(&lib.SourceData{
 				Filename: "/script.js",
 				Data: []byte(`

lib/metrics/metrics.go

@@ -24,6 +24,8 @@ import (
 	"github.com/loadimpact/k6/stats"
 )
 
+//TODO: refactor this, using non thread-safe global variables seems like a bad idea for various reasons...
+
 var (
 	// Engine-emitted.
 	VUs               = stats.New("vus", stats.Gauge)