Port 'Don't disable HTTP keep-alives for remote storage connections.' to 2.0 (see #3173)

Removes configurability introduced in #3160 in favour of hard-coding,
per advice from @brian-brazil.

storage/remote/client.go

@@ -55,7 +55,7 @@ type ClientConfig struct {
 
 // NewClient creates a new Client.
 func NewClient(index int, conf *ClientConfig) (*Client, error) {
-	httpClient, err := httputil.NewClientFromConfig(conf.HTTPClientConfig, "remote_storage")
+	httpClient, err := httputil.NewClientFromConfigAndOptions(conf.HTTPClientConfig, "remote_storage", false)
 	if err != nil {
 		return nil, err
 	}

util/httputil/client.go

@@ -34,6 +34,12 @@ func newClient(rt http.RoundTripper) *http.Client {
 // NewClientFromConfig returns a new HTTP client configured for the
 // given config.HTTPClientConfig. The name is used as go-conntrack metric label.
 func NewClientFromConfig(cfg config.HTTPClientConfig, name string) (*http.Client, error) {
+	return NewClientFromConfigAndOptions(cfg, name, true)
+}
+
+// NewClientFromConfigAndOptions returns a new HTTP client configured for the
+// given config.HTTPClientConfig. The name is used as go-conntrack metric label.
+func NewClientFromConfigAndOptions(cfg config.HTTPClientConfig, name string, disableKeepAlives bool) (*http.Client, error) {
 	tlsConfig, err := NewTLSConfig(cfg.TLSConfig)
 	if err != nil {
 		return nil, err
@@ -43,7 +49,7 @@ func NewClientFromConfig(cfg config.HTTPClientConfig, name string) (*http.Client
 	var rt http.RoundTripper = &http.Transport{
 		Proxy:              http.ProxyURL(cfg.ProxyURL.URL),
 		MaxIdleConns:       20000,
-		DisableKeepAlives:  false,
+		DisableKeepAlives:  disableKeepAlives,
 		TLSClientConfig:    tlsConfig,
 		DisableCompression: true,
 		// 5 minutes is typically above the maximum sane scrape interval. So we can