Re-enable HTTP/2 (prometheus#325)

* Re-enable HTTP/2

HTTP/2 has been tested and validated by some of our users. It is time to
bring it back.

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

config/http_config.go

@@ -393,17 +393,18 @@ func NewRoundTripperFromConfig(cfg HTTPClientConfig, name string, optFuncs ...HT
 			DialContext:           dialContext,
 			DialTLSContext:        opts.dialTLSContextFunc,
 		}
-		if opts.http2Enabled || os.Getenv("PROMETHEUS_COMMON_ENABLE_HTTP2") != "" {
-			// HTTP/2 support is golang has many problematic cornercases where
+		if opts.http2Enabled && os.Getenv("PROMETHEUS_COMMON_DISABLE_HTTP2") == "" {
+			// 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
 			// https://github.com/golang/go/issues/39337
 			// https://github.com/golang/go/issues/39750
 
-			// Enable HTTP2 if the environment variable
-			// PROMETHEUS_COMMON_ENABLE_HTTP2 is set.
-			// This is a temporary workaround so that users can safely test this
-			// and validate that HTTP2 can be enabled Prometheus-Wide again.
+			// Do not enable HTTP2 if the environment variable
+			// PROMETHEUS_COMMON_DISABLE_HTTP2 is set to a non-empty value.
+			// This allows users to easily disable HTTP2 in case they run into
+			// issues again, but will be removed once we are confident that
+			// things work as expected.
 
 			http2t, err := http2.ConfigureTransports(rt.(*http.Transport))
 			if err != nil {