Add feature flag state metrics (#340)

Author: atennak1

src/featureFlag/FeatureFlagProvider.ts

@@ -1,7 +1,8 @@
 import { writeFileSync } from 'fs';
 import { join } from 'path';
 import { LoggerFactory } from '../telemetry/LoggerFactory';
-import { Measure } from '../telemetry/TelemetryDecorator';
+import { ScopedTelemetry } from '../telemetry/ScopedTelemetry';
+import { Measure, Telemetry } from '../telemetry/TelemetryDecorator';
 import { Closeable } from '../utils/Closeable';
 import { AwsEnv } from '../utils/Environment';
 import { readFileIfExists } from '../utils/File';
@@ -12,6 +13,9 @@ import { FeatureFlagSupplier, FeatureFlagConfigKey, TargetedFeatureFlagConfigKey
 const log = LoggerFactory.getLogger('FeatureFlagProvider');
 
 export class FeatureFlagProvider implements Closeable {
+    @Telemetry()
+    private readonly telemetry!: ScopedTelemetry;
+
     private config: unknown;
     private readonly supplier: FeatureFlagSupplier;
 
@@ -44,6 +48,7 @@ export class FeatureFlagProvider implements Closeable {
             5 * 60 * 1000,
         );
 
+        this.registerGauges();
         this.log();
     }
 
@@ -81,6 +86,14 @@ export class FeatureFlagProvider implements Closeable {
         );
     }
 
+    private registerGauges() {
+        for (const [key, flag] of this.supplier.featureFlags.entries()) {
+            this.telemetry.registerGaugeProvider(`featureFlag.${key}`, () => (flag.isEnabled() ? 1 : 0), {
+                description: `State of ${key} feature flag`,
+            });
+        }
+    }
+
     close() {
         this.supplier.close();
         clearInterval(this.timeout);

tst/unit/featureFlag/FeatureFlagProvider.test.ts

@@ -1,7 +1,9 @@
 import { readFileSync } from 'fs';
 import { join } from 'path';
-import { describe, it, expect } from 'vitest';
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
 import { FeatureFlagConfigSchema } from '../../../src/featureFlag/FeatureFlagBuilder';
+import { FeatureFlagProvider } from '../../../src/featureFlag/FeatureFlagProvider';
+import { ScopedTelemetry } from '../../../src/telemetry/ScopedTelemetry';
 
 describe('FeatureFlagProvider', () => {
     it('can parse feature flags', () => {
@@ -15,4 +17,42 @@ describe('FeatureFlagProvider', () => {
             expect(FeatureFlagConfigSchema.parse(JSON.parse(file))).toBeDefined();
         });
     });
+
+    describe('gauge registration', () => {
+        let provider: FeatureFlagProvider;
+        let registerGaugeProviderSpy: ReturnType<typeof vi.spyOn>;
+
+        beforeEach(() => {
+            registerGaugeProviderSpy = vi.spyOn(ScopedTelemetry.prototype, 'registerGaugeProvider');
+        });
+
+        afterEach(() => {
+            provider?.close();
+            vi.restoreAllMocks();
+        });
+
+        it('registers gauges for each feature flag', () => {
+            provider = new FeatureFlagProvider(
+                () => Promise.resolve({ features: { Constants: { enabled: true } } }),
+                join(__dirname, '..', '..', '..', 'assets', 'featureFlag', 'alpha.json'),
+            );
+
+            expect(registerGaugeProviderSpy).toHaveBeenCalledWith(
+                'featureFlag.Constants',
+                expect.any(Function),
+                expect.objectContaining({ description: 'State of Constants feature flag' }),
+            );
+        });
+
+        it('gauge provider reflects current flag state', () => {
+            provider = new FeatureFlagProvider(
+                () => Promise.resolve({ features: { Constants: { enabled: false } } }),
+                join(__dirname, '..', '..', '..', 'assets', 'featureFlag', 'alpha.json'),
+            );
+
+            const gaugeProvider = registerGaugeProviderSpy.mock.calls[0][1] as () => number;
+            // Alpha config has Constants disabled
+            expect(gaugeProvider()).toBe(0);
+        });
+    });
 });