state: implement SignedDataSource on MetricsRequest message

state/sign.go

@@ -6,3 +6,10 @@ package state
 func (m NetmapRequest) SignedData() ([]byte, error) {
 	return make([]byte, 0), nil
 }
+
+// SignedData returns payload bytes of the request.
+//
+// Always returns empty slice.
+func (m MetricsRequest) SignedData() ([]byte, error) {
+	return make([]byte, 0), nil
+}

state/sign_test.go

@@ -27,6 +27,11 @@ func TestRequestSign(t *testing.T) {
 				return new(NetmapRequest)
 			},
 		},
+		{ // MetricsRequest
+			constructor: func() sigType {
+				return new(MetricsRequest)
+			},
+		},
 	}
 
 	for _, item := range items {