lib/metricsql: add example for ExpandWithExprs

lib/metricsql/utils_example_test.go

@@ -0,0 +1,28 @@
+package metricsql_test
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/metricsql"
+)
+
+func ExampleExpandWithExprs() {
+	// mql can contain arbitrary MetricsQL extensions - see https://github.com/VictoriaMetrics/VictoriaMetrics/wiki/ExtendedPromQL
+	mql := `WITH (
+		commonFilters = {job="$job", instance="$instance"},
+		f(a, b) = 100*(a/b),
+	)
+	f(disk_free_bytes{commonFilters}, disk_total_bytes{commonFilters})`
+
+	// Convert mql to PromQL
+	pql, err := metricsql.ExpandWithExprs(mql)
+	if err != nil {
+		log.Fatalf("cannot expand with expressions: %s", err)
+	}
+
+	fmt.Printf("%s\n", pql)
+
+	// Output:
+	// 100 * (disk_free_bytes{job="$job", instance="$instance"} / disk_total_bytes{job="$job", instance="$instance"})
+}