Moving the query building logic, adding first unit test.

Moving it into its own type to better organize and separate concerns. Also makes unit testing easier.

pkg/custom-provider/external_metric_query_builder_test.go

@@ -0,0 +1,23 @@
+package provider
+
+import (
+	"testing"
+
+	"k8s.io/apimachinery/pkg/labels"
+	"k8s.io/apimachinery/pkg/selection"
+)
+
+var queryBuilder = NewExternalMetricQueryBuilder()
+
+func TestBuildPrometheusQuery(t *testing.T) {
+	fakeSelector := labels.NewSelector()
+	requirement, _ := labels.NewRequirement("queue_name", selection.Equals, []string{"processing"})
+	fakeSelector = fakeSelector.Add(*requirement)
+
+	result := queryBuilder.BuildPrometheusQuery("default", "queue_length", fakeSelector)
+
+	expectedResult := "queue_length{namespace=\"default\", queue_name=\"processing\"}"
+	if result != expectedResult {
+		t.Errorf("Incorrect query generated. Expected: %s | Actual %s", result, expectedResult)
+	}
+}