Add copyright header

src/metrics.ts

+/*
+Copyright 2019 matrix-appservice-discord
+
+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.
+*/
+
 import { PrometheusMetrics, Bridge } from "matrix-appservice-bridge";
 import { Gauge, Counter, Histogram } from "prom-client";
 import { Log } from "./log";