feat: Normalized server logging (#2567)

* feat: Normalize logging

* Remove scattered console.error + Sentry.captureException

* Remove mention of debug

* cleanup dev output

* Edge cases, docs

* Refactor: Move logger, metrics, sentry under 'logging' folder.
Trying to reduce the amount of things under generic 'utils'

* cleanup, last few console calls

server/logging/metrics.js

@@ -0,0 +1,51 @@
+// @flow
+import ddMetrics from "datadog-metrics";
+
+class Metrics {
+  enabled: boolean = !!process.env.DD_API_KEY;
+
+  constructor() {
+    if (!this.enabled) {
+      return;
+    }
+
+    ddMetrics.init({
+      apiKey: process.env.DD_API_KEY,
+      prefix: "outline.",
+      defaultTags: [`env:${process.env.DD_ENV || process.env.NODE_ENV}`],
+    });
+  }
+
+  gauge(key: string, value: number, tags?: string[]): void {
+    if (!this.enabled) {
+      return;
+    }
+
+    return ddMetrics.gauge(key, value, tags);
+  }
+
+  gaugePerInstance(key: string, value: number, tags?: string[] = []): void {
+    if (!this.enabled) {
+      return;
+    }
+
+    const instanceId = process.env.INSTANCE_ID || process.env.HEROKU_DYNO_ID;
+    if (!instanceId) {
+      throw new Error(
+        "INSTANCE_ID or HEROKU_DYNO_ID must be set when using DataDog"
+      );
+    }
+
+    return ddMetrics.gauge(key, value, [...tags, `instance:${instanceId}`]);
+  }
+
+  increment(key: string, tags?: { [string]: string }): void {
+    if (!this.enabled) {
+      return;
+    }
+
+    return ddMetrics.increment(key, tags);
+  }
+}
+
+export default new Metrics();