Prevent from attempting to create metrics with unspecified MetricKind (#305)

* Fixes errors returned when attempting to create metrics with unspecified metric kind.

Author: ygonzal6

metrics.go

@@ -261,6 +261,11 @@ func (se *statsExporter) createMetricDescriptorFromMetric(ctx context.Context, m
 		return err
 	}
 
+	// ignore
+	if inMD.MetricKind == googlemetricpb.MetricDescriptor_METRIC_KIND_UNSPECIFIED {
+		return nil
+	}
+
 	if err = se.createMetricDescriptor(ctx, inMD); err != nil {
 		return err
 	}

metrics_test.go

@@ -127,7 +127,9 @@ func TestMetricToCreateTimeSeriesRequest(t *testing.T) {
 											Count:    1,
 											Exemplar: &metricdata.Exemplar{Value: 11.9, Timestamp: startTime, Attachments: map[string]interface{}{"key": "value"}},
 										},
-										{}, {}, {},
+										{},
+										{},
+										{},
 									},
 									BucketOptions: &metricdata.BucketOptions{
 										Bounds: []float64{10, 20, 30, 40},
@@ -463,7 +465,8 @@ func TestMetricsToMonitoringMetrics_fromProtoPoint(t *testing.T) {
 						{},
 						{
 							Count:    1,
-							Exemplar: &metricdata.Exemplar{Value: 11.9, Timestamp: startTime, Attachments: map[string]interface{}{"SpanContext": spanCtx}}},
+							Exemplar: &metricdata.Exemplar{Value: 11.9, Timestamp: startTime, Attachments: map[string]interface{}{"SpanContext": spanCtx}},
+						},
 						{},
 						{},
 						{},
@@ -945,7 +948,7 @@ func TestResourceByDescriptor(t *testing.T) {
 		},
 	}
 
-	var se = &statsExporter{
+	se := &statsExporter{
 		o: Options{
 			ProjectID:            "foo",
 			ResourceByDescriptor: getResourceByDescriptor,