Add vps usage query example

examples/usage.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"fmt"
+	"github.com/transip/gotransip/v6"
+	"github.com/transip/gotransip/v6/vps"
+	"log"
+	"time"
+)
+
+func main() {
+	// Create a new client with the default demo client config, using the demo token
+	client, err := gotransip.NewClient(gotransip.DemoClientConfiguration)
+	if err != nil {
+		panic(err)
+	}
+
+	vpsRepo := vps.Repository{Client: client}
+
+	// Create a usage period with which you can query Vps usage for the last 10 minutes
+	log.Println("Getting a list of cpu usage for the last 10 minutes")
+	period := vps.UsagePeriod{
+		TimeStart: time.Now().Unix() - 600,
+		TimeEnd:   time.Now().Unix(),
+	}
+
+	// Query with our usage period for the Vps 'transipdemo-vps4'
+	usage, err := vpsRepo.GetUsage("transipdemo-vps4", []vps.UsageType{vps.UsageTypeCPU}, period)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Printf("%+v\n", usage)
+}