gateway: add cap to detect metadata support

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
(cherry picked from commit de5af7d6c2)

frontend/gateway/pb/caps.go

@@ -32,6 +32,9 @@ const (
 	// CapFrontendInputs is a capability to request frontend inputs from the
 	// LLBBridge GRPC server.
 	CapFrontendInputs apicaps.CapID = "frontend.inputs"
+
+	// CapGatewaySolveMetadata can be used to check if solve calls from gateway reliably return metadata
+	CapGatewaySolveMetadata apicaps.CapID = "gateway.solve.metadata"
 )
 
 func init() {
@@ -126,4 +129,11 @@ func init() {
 		Enabled: true,
 		Status:  apicaps.CapStatusExperimental,
 	})
+
+	Caps.Init(apicaps.Cap{
+		ID:      CapGatewaySolveMetadata,
+		Name:    "gateway metadata",
+		Enabled: true,
+		Status:  apicaps.CapStatusExperimental,
+	})
 }