Add User-Agent header

This makes it easier on server admins to differentiate requests, block
us if they feel our usage is inappropriate and informs them how they can
reach out to us to resolve any issues.

data.go

@@ -11,7 +11,8 @@ import (
 )
 
 const (
-	source = "https://www.vattenfall.se/api/price/spot/pricearea/%s/%s/%s"
+	source    = "https://www.vattenfall.se/api/price/spot/pricearea/%s/%s/%s"
+	userAgent = "vattenfall-exporter/%s (+https://github.com/daenney/vattenfall)"
 )
 
 type Data struct {
@@ -87,7 +88,15 @@ func fetchFromURL(date time.Time, region string) ([]byte, error) {
 		date.Format("2006-01-02"),
 		region,
 	)
-	resp, err := http.Get(res)
+
+	req, err := http.NewRequest("GET", res, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	req.Header.Set("User-Agent", fmt.Sprintf(userAgent, version))
+
+	resp, err := http.DefaultClient.Do(req)
 	if err != nil {
 		return nil, fmt.Errorf("failed to fetch %s: %w", res, err)
 	}