Add http transport reader

Copy code from GetJSONFromURL into a http reader constructor

transport/http.go

@@ -50,3 +50,47 @@ func GetJSONFromURL(url string, timeout time.Duration, target interface{}) error
 
 	return json.NewDecoder(reader).Decode(target)
 }
+
+type httpReader struct {
+	URL     string
+	Timeout time.Duration
+}
+
+func newHTTPReader(url string, timeout time.Duration) (*io.ReadCloser, error) {
+	hr := httpReader{URL: url, Timeout: timeout}
+
+	log.Infof("GET %s timeout=%s", hr.URL, hr.Timeout)
+
+	c := &http.Client{
+		Timeout: timeout,
+	}
+
+	req, err := http.NewRequest("GET", hr.URL, nil)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Add("Accept-Encoding", "gzip")
+	resp, err := c.Do(req)
+	if err != nil {
+		return nil, err
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("Request to Alertmanager failed with %s", resp.Status)
+	}
+
+	defer resp.Body.Close()
+
+	var reader io.ReadCloser
+	switch resp.Header.Get("Content-Encoding") {
+	case "gzip":
+		reader, err = gzip.NewReader(resp.Body)
+		if err != nil {
+			return nil, fmt.Errorf("Failed to decode gzipped content: %s", err.Error())
+		}
+		defer reader.Close()
+	default:
+		reader = resp.Body
+	}
+	return &reader, nil
+}