Scaffold monorepo dev setup

packages/observability/package.json

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+{
+  "name": "@islandflow/observability",
+  "private": true,
+  "type": "module",
+  "exports": {
+    ".": "./src/index.ts"
+  }
+}

packages/observability/src/index.ts

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+export * from "./logger";
+export * from "./metrics";

packages/observability/src/logger.ts

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+export type LogLevel = "debug" | "info" | "warn" | "error";
+
+export type LogContext = Record<string, unknown>;
+
+export type LogRecord = LogContext & {
+  level: LogLevel;
+  service: string;
+  msg: string;
+  ts: string;
+};
+
+export type LoggerFn = (msg: string, context?: LogContext) => void;
+
+export type Logger = {
+  debug: LoggerFn;
+  info: LoggerFn;
+  warn: LoggerFn;
+  error: LoggerFn;
+};
+
+export type LoggerOptions = {
+  service: string;
+  now?: () => string;
+  sink?: (record: LogRecord) => void;
+};
+
+const defaultSink = (record: LogRecord) => {
+  console.log(JSON.stringify(record));
+};
+
+export const createLogger = ({
+  service,
+  now = () => new Date().toISOString(),
+  sink = defaultSink
+}: LoggerOptions): Logger => {
+  const write = (level: LogLevel, msg: string, context?: LogContext) => {
+    const record: LogRecord = {
+      level,
+      service,
+      msg,
+      ts: now(),
+      ...(context ?? {})
+    };
+
+    sink(record);
+  };
+
+  return {
+    debug: (msg, context) => write("debug", msg, context),
+    info: (msg, context) => write("info", msg, context),
+    warn: (msg, context) => write("warn", msg, context),
+    error: (msg, context) => write("error", msg, context)
+  };
+};

packages/observability/src/metrics.ts

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+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)
+  };
+};

packages/observability/tsconfig.json

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+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "types": []
+  },
+  "include": ["src/**/*.ts"]
+}