Scaffold monorepo dev setup

packages/observability/src/metrics.ts

@@ -0,0 +1,51 @@
+export type MetricType = "counter" | "gauge" | "timing";
+
+export type MetricTags = Record<string, string>;
+
+export type MetricRecord = {
+  name: string;
+  type: MetricType;
+  value: number;
+  ts: number;
+  service?: string;
+  tags?: MetricTags;
+};
+
+export type MetricsEmitter = (record: MetricRecord) => void;
+
+export type MetricsOptions = {
+  service?: string;
+  emit?: MetricsEmitter;
+  now?: () => number;
+};
+
+export type Metrics = {
+  count: (name: string, value?: number, tags?: MetricTags) => void;
+  gauge: (name: string, value: number, tags?: MetricTags) => void;
+  timing: (name: string, value: number, tags?: MetricTags) => void;
+};
+
+const noopEmit: MetricsEmitter = () => {};
+
+export const createMetrics = ({
+  service,
+  emit = noopEmit,
+  now = () => Date.now()
+}: MetricsOptions = {}): Metrics => {
+  const write = (type: MetricType, name: string, value: number, tags?: MetricTags) => {
+    emit({
+      name,
+      type,
+      value,
+      tags,
+      service,
+      ts: now()
+    });
+  };
+
+  return {
+    count: (name, value = 1, tags) => write("counter", name, value, tags),
+    gauge: (name, value, tags) => write("gauge", name, value, tags),
+    timing: (name, value, tags) => write("timing", name, value, tags)
+  };
+};