Retain SwiftNIO copyright header (#10)

motivation: retain copyrights

changes:
* add notice file with references to derived work
* add swift-nio license header to locks

NOTICE.txt

@@ -0,0 +1,43 @@
+
+                            The SwiftMetrics Project
+                            ========================
+
+Please visit the SwiftMetrics web site for more information:
+
+  * https://github.com/apple/swift-metrics
+
+Copyright 2018, 2019 The SwiftMetrics Project
+
+The SwiftMetrics Project licenses this file to you 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:
+
+  https://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.
+
+Also, please refer to each LICENSE.<component>.txt file, which is located in
+the 'license' directory of the distribution file, for the license terms of the
+components that this product depends on.
+
+-------------------------------------------------------------------------------
+
+This product contains a derivation of the Tony Stone's 'process_test_files.rb'.
+
+  * LICENSE (Apache License 2.0):
+    * https://www.apache.org/licenses/LICENSE-2.0
+  * HOMEPAGE:
+    * https://codegists.com/snippet/ruby/generate_xctest_linux_runnerrb_tonystone_ruby
+
+---
+
+This product contains a derivation of SwiftNIO locks.
+
+  * LICENSE (Apache License 2.0):
+    * https://www.apache.org/licenses/LICENSE-2.0
+  * HOMEPAGE:
+    * https://github.com/apple/swift-nio

Sources/CoreMetrics/Locks.swift

@@ -12,6 +12,20 @@
 //
 //===----------------------------------------------------------------------===//
 
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftNIO open source project
+//
+// Copyright (c) 2017-2018 Apple Inc. and the SwiftNIO project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
     import Darwin
 #else