feat: add last fetch and last update

README.md

@@ -506,6 +506,15 @@ Connection: keep-alive
 Keep-Alive: timeout=5
 ```
 
+**Prometheus endpoint**
+
+The proxy has a prometheus metrics endpoint available at http://localhost:3000/proxy/internal-backstage/prometheus
+With the following metrics 
+* `unleash_proxy_up` a [counter](https://prometheus.io/docs/concepts/metric_types/#counter) which is set to 1 when proxy is running
+* `last_metrics_update_epoch_timestamp_ms` a [gauge](https://prometheus.io/docs/concepts/metric_types/#gauge) set to the epoch timestamp in ms when the proxy last received a feature update
+* `last_metrics_fetch_epoch_timestamp_ms` a [gauge](https://prometheus.io/docs/concepts/metric_types/#gauge) set to the epoch timestamp in ms when the proxy last checked for updates
+
+
 ### Run with Node.js:
 
 **STEP 1: Install dependency**

package.json

@@ -44,6 +44,7 @@
     "express": "^4.18.2",
     "json-schema-to-ts": "^2.3.0",
     "openapi-types": "^11.0.0",
+    "prom-client": "^15.1.3",
     "qs": "^6.9.7",
     "type-is": "^1.6.18",
     "unleash-client": "^6.1.1"
@@ -54,7 +55,7 @@
     "@types/compression": "^1.7.2",
     "@types/cors": "^2.8.12",
     "@types/express": "^4.17.13",
-    "@types/jest": "^27.5.0",
+    "@types/jest": "^29.5.12",
     "@types/node": "18.7.14",
     "@types/supertest": "^2.0.12",
     "@types/type-is": "^1.6.3",
@@ -67,12 +68,12 @@
     "eslint-config-prettier": "^8.5.0",
     "eslint-plugin-import": "^2.26.0",
     "eslint-plugin-prettier": "^4.0.0",
-    "jest": "^28.0.3",
+    "jest": "^29.7.0",
     "prettier": "^2.6.2",
     "semver": "^7.3.7",
     "supertest": "^6.2.3",
-    "ts-jest": "^28.0.1",
-    "ts-node-dev": "^1.1.8",
+    "ts-jest": "^29.2.4",
+    "ts-node-dev": "^2.0.0",
     "typescript": "^4.6.4"
   },
   "prettier": {

src/client.ts

@@ -1,10 +1,11 @@
 import EventEmitter from 'events';
-import { Context, Unleash, Variant } from 'unleash-client';
+import { Context, Unleash, UnleashEvents, Variant } from 'unleash-client';
 import { FeatureInterface } from 'unleash-client/lib/feature';
 import Metrics from 'unleash-client/lib/metrics';
 import { getDefaultVariant } from 'unleash-client/lib/variant';
 import { IProxyConfig } from './config';
 import { Logger } from './logger';
+import { lastMetricsFetch, lastMetricsUpdate } from './prom-metrics';
 
 export type FeatureToggleStatus = {
     name: string;
@@ -77,6 +78,14 @@ class Client extends EventEmitter implements IClient {
             this.ready = true;
             this.metrics.start();
         });
+        this.unleash.on(UnleashEvents.Unchanged, () => {
+            lastMetricsFetch.set(new Date().getTime());
+        });
+        this.unleash.on(UnleashEvents.Changed, () => {
+            const updatedAt = new Date().getTime();
+            lastMetricsFetch.set(updatedAt);
+            lastMetricsUpdate.set(updatedAt);
+        });
     }
 
     setUnleashApiToken(unleashApiToken: string): void {

src/prom-metrics/createCounter.ts

@@ -0,0 +1,64 @@
+import { Counter as PromCounter, type CounterConfiguration } from 'prom-client';
+
+/**
+ * A wrapped instance of prom-client's Counter, overriding some of its methods for enhanced functionality and type-safety.
+ */
+export type Counter<T extends string = string> = {
+    counter: PromCounter<T>;
+    labels: (labels: Record<T, string | number>) => PromCounter.Internal;
+    inc: (value?: number | undefined) => void;
+    increment: (labels: Record<T, string | number>, value?: number) => void;
+};
+
+/**
+ * Creates a wrapped instance of prom-client's Counter, overriding some of its methods for enhanced functionality and type-safety.
+ *
+ * @param options - The configuration options for the Counter, as defined in prom-client's CounterConfiguration.
+ *               See prom-client documentation for detailed options: https://github.com/siimon/prom-client#counter
+ * @returns An object containing the wrapped Counter instance and custom methods.
+ */
+export const createCounter = <T extends string>(
+    options: CounterConfiguration<T>,
+): Counter<T> => {
+    /**
+     * The underlying instance of prom-client's Counter.
+     */
+    const counter = new PromCounter<T>(options);
+
+    /**
+     * Applies given labels to the counter. Labels are key-value pairs.
+     * This method wraps the original Counter's labels method for additional type-safety, requiring all configured labels to be specified.
+     *
+     * @param labels - An object where keys are label names and values are the label values.
+     * @returns The Counter instance with the applied labels, allowing for method chaining.
+     */
+    // eslint-disable-next-line @typescript-eslint/no-shadow
+    const labels = (labels: Record<T, string | number>) =>
+        counter.labels(labels);
+
+    /**
+     * Increments the counter by a specified value or by 1 if no value is provided.
+     * Wraps the original Counter's inc method.
+     *
+     * @param value - (Optional) The value to increment the counter by. If not provided, defaults to 1.
+     */
+    const inc = (value?: number | undefined) => counter.inc(value);
+
+    /**
+     * A convenience method that combines setting labels and incrementing the counter.
+     * Useful for incrementing with labels in a single call.
+     *
+     * @param labels - An object where keys are label names and values are the label values.
+     * @param value - (Optional) The value to increment the counter by. If not provided, defaults to 1.
+     */
+    // eslint-disable-next-line @typescript-eslint/no-shadow
+    const increment = (labels: Record<T, string | number>, value?: number) =>
+        counter.labels(labels).inc(value);
+
+    return {
+        counter,
+        labels,
+        inc,
+        increment,
+    };
+};

src/prom-metrics/createGauge.ts

@@ -0,0 +1,51 @@
+import { Gauge as PromGauge, type GaugeConfiguration } from 'prom-client';
+
+/**
+ * A wrapped instance of prom-client's Gauge, overriding some of its methods for enhanced functionality and type-safety.
+ */
+export type Gauge<T extends string = string> = {
+    gauge: PromGauge<T>;
+    labels: (labels: Record<T, string | number>) => PromGauge.Internal<T>;
+    reset: () => void;
+    set: (value: number) => void;
+};
+
+export const createGauge = <T extends string>(
+    options: GaugeConfiguration<T>,
+): Gauge<T> => {
+    /**
+     * The underlying instance of prom-client's Gauge.
+     */
+    const gauge = new PromGauge(options);
+
+    /**
+     * Applies given labels to the gauge. Labels are key-value pairs.
+     * This method wraps the original Gauge's labels method for additional type-safety, requiring all configured labels to be specified.
+     *
+     * @param labels - An object where keys are label names and values are the label values.
+     * @returns The Gauge instance with the applied labels, allowing for method chaining.
+     */
+    // eslint-disable-next-line @typescript-eslint/no-shadow
+    const labels = (labels: Record<T, string | number>) => gauge.labels(labels);
+
+    /**
+     * Resets the gauge value.
+     * Wraps the original Gauge's reset method.
+     */
+    const reset = () => gauge.reset();
+
+    /**
+     * Sets the gauge to a specified value.
+     * Wraps the original Gauge's set method.
+     *
+     * @param value - The value to set the gauge to.
+     */
+    const set = (value: number) => gauge.set(value);
+
+    return {
+        gauge,
+        labels,
+        reset,
+        set,
+    };
+};

src/prom-metrics/index.ts

@@ -0,0 +1,20 @@
+import { createGauge, type Gauge } from './createGauge';
+import { createCounter } from './createCounter';
+
+export * from './createCounter';
+export * from './createGauge';
+
+export const lastMetricsUpdate: Gauge = createGauge({
+    name: 'last_metrics_update_epoch_timestamp_ms',
+    help: 'An epoch timestamp (in milliseconds) set to when our unleash-client last got an update from upstream Unleash',
+});
+
+export const lastMetricsFetch: Gauge = createGauge({
+    name: 'last_metrics_fetch_epoch_timestamp_ms',
+    help: 'An epoch timestamp (in milliseconds) set to when our unleash-client last checked (regardless if there was an update or not)',
+});
+
+createCounter({
+    name: 'unleash_proxy_up',
+    help: 'Indication that the service is up.',
+}).inc(1);