greentea: Add missing license header

Add missing license header to greentea_metrics.h

features/frameworks/greentea-client/greentea-client/greentea_metrics.h

@@ -1,3 +1,21 @@
+/*
+ * mbed Microcontroller Library
+ * Copyright (c) 2006-2019 ARM Limited
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
 #include "platform/mbed_toolchain.h"
 
 /** \addtogroup frameworks */