fix http host header

Override the HTTP request header `Host` to the original target host, after resolving it's IP and creating the HTTP request context.

Signed-off-by: Kevin Weis <Kevin.Weis@plan.de>

prober/http.go

@@ -392,7 +392,6 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 		httpConfig.Method = "GET"
 	}
 
-	origHost := targetURL.Host
 	if ip != nil {
 		// Replace the host field in the URL with the IP we resolved.
 		if targetPort == "" {
@@ -430,7 +429,7 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 		level.Error(logger).Log("msg", "Error creating request", "err", err)
 		return
 	}
-	request.Host = origHost
+	request.Host = targetHost
 	request = request.WithContext(ctx)
 
 	for key, value := range httpConfig.Headers {