feat: DataDog metrics (#2228)

* wip

* chore: Change event names, add additional events

* fix: Not counting connect events

server/utils/metrics.js

@@ -0,0 +1,26 @@
+// @flow
+import metrics from "datadog-metrics";
+
+if (process.env.DD_API_KEY) {
+  metrics.init({
+    apiKey: process.env.DD_API_KEY,
+    prefix: "outline.",
+    defaultTags: [`env:${process.env.DD_ENV || process.env.NODE_ENV}`],
+  });
+}
+
+export function gauge(key: string, value: number): void {
+  if (!process.env.DD_API_KEY) {
+    return;
+  }
+
+  return metrics.gauge(key, value);
+}
+
+export function increment(key: string, tags?: { [string]: string }): void {
+  if (!process.env.DD_API_KEY) {
+    return;
+  }
+
+  return metrics.increment(key, tags);
+}