expose scrape.userAgentHeader like remote.UserAgent

Signed-off-by: Robert Fratto <robertfratto@gmail.com>

scrape/scrape.go

@@ -721,7 +721,7 @@ var errBodySizeLimit = errors.New("body size limit exceeded")
 
 const acceptHeader = `application/openmetrics-text; version=0.0.1,text/plain;version=0.0.4;q=0.5,*/*;q=0.1`
 
-var userAgentHeader = fmt.Sprintf("Prometheus/%s", version.Version)
+var UserAgent = fmt.Sprintf("Prometheus/%s", version.Version)
 
 func (s *targetScraper) scrape(ctx context.Context, w io.Writer) (string, error) {
 	if s.req == nil {
@@ -731,7 +731,7 @@ func (s *targetScraper) scrape(ctx context.Context, w io.Writer) (string, error)
 		}
 		req.Header.Add("Accept", acceptHeader)
 		req.Header.Add("Accept-Encoding", "gzip")
-		req.Header.Set("User-Agent", userAgentHeader)
+		req.Header.Set("User-Agent", UserAgent)
 		req.Header.Set("X-Prometheus-Scrape-Timeout-Seconds", strconv.FormatFloat(s.timeout.Seconds(), 'f', -1, 64))
 
 		s.req = req