Gitaly feature flags are broken

In rails we convert _ to - in flag names. In Gitaly, we forget to convert back.

This fixes it:

diff --git a/internal/metadata/featureflag/grpc_header.go b/internal/metadata/featureflag/grpc_header.go
index 94940c9c..e5e450c9 100644
--- a/internal/metadata/featureflag/grpc_header.go
+++ b/internal/metadata/featureflag/grpc_header.go
@@ -3,6 +3,7 @@ package featureflag
 import (
 	"context"
 	"fmt"
+	"strings"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"google.golang.org/grpc/metadata"
@@ -51,5 +52,5 @@ func IsDisabled(ctx context.Context, flag string) bool {
 
 // HeaderKey returns the feature flag key to be used in the metadata map
 func HeaderKey(flag string) string {
-	return fmt.Sprintf("gitaly-feature-%s", flag)
+	return fmt.Sprintf("gitaly-feature-%s", strings.ReplaceAll(flag, "_", "-"))
 }

The MR that fixes this should also test this behavior.