Make HTTP2 user visible

We still get reports from users that have issues with HTTP2. Let's make
it explicitly possible to disable it.

Because we previously introduced FollowRedirects, most lib users already
use our defaults.

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

config/http_config.go

@@ -28,7 +28,6 @@ import (
 	"net"
 	"net/http"
 	"net/url"
-	"os"
 	"strings"
 	"sync"
 	"time"
@@ -43,6 +42,7 @@ import (
 // DefaultHTTPClientConfig is the default HTTP client configuration.
 var DefaultHTTPClientConfig = HTTPClientConfig{
 	FollowRedirects: true,
+	EnableHTTP2:     true,
 }
 
 // defaultHTTPClientOptions holds the default HTTP client options.
@@ -198,6 +198,10 @@ type HTTPClientConfig struct {
 	// The omitempty flag is not set, because it would be hidden from the
 	// marshalled configuration when set to false.
 	FollowRedirects bool `yaml:"follow_redirects" json:"follow_redirects"`
+	// EnableHTTP2 specifies whether the client should configure HTTP2.
+	// The omitempty flag is not set, because it would be hidden from the
+	// marshalled configuration when set to false.
+	EnableHTTP2 bool `yaml:"enable_http2" json:"enable_http2"`
 }
 
 // SetDirectory joins any relative file paths with dir.
@@ -401,7 +405,7 @@ func NewRoundTripperFromConfig(cfg HTTPClientConfig, name string, optFuncs ...HT
 			ExpectContinueTimeout: 1 * time.Second,
 			DialContext:           dialContext,
 		}
-		if opts.http2Enabled && os.Getenv("PROMETHEUS_COMMON_DISABLE_HTTP2") == "" {
+		if opts.http2Enabled && cfg.EnableHTTP2 {
 			// HTTP/2 support is golang had many problematic cornercases where
 			// dead connections would be kept and used in connection pools.
 			// https://github.com/golang/go/issues/32388

config/testdata/http.conf.good.yml

@@ -2,3 +2,4 @@ basic_auth:
   username: username
   password: "mysecret"
 proxy_url: "http://remote.host"
+enable_http2: true