Add thresholds to attribution build (GoogleChrome#340)

* Add thresholds to attribution build

* Add tests

src/attribution.ts

@@ -21,4 +21,11 @@ export {onINP} from './attribution/onINP.js';
 export {onLCP} from './attribution/onLCP.js';
 export {onTTFB} from './attribution/onTTFB.js';
 
+export {CLSThresholds} from './onCLS.js';
+export {FCPThresholds} from './onFCP.js';
+export {FIDThresholds} from './onFID.js';
+export {INPThresholds} from './onINP.js';
+export {LCPThresholds} from './onLCP.js';
+export {TTFBThresholds} from './onTTFB.js';
+
 export * from './types.js';

test/unit/attribution-test.js

@@ -0,0 +1,33 @@
+import {describe, it} from 'node:test';
+import assert from 'assert';
+import {
+  onCLS,
+  onFCP,
+  onFID,
+  onINP,
+  onLCP,
+  onTTFB,
+  CLSThresholds,
+  FCPThresholds,
+  FIDThresholds,
+  INPThresholds,
+  LCPThresholds,
+  TTFBThresholds,
+} from 'web-vitals/attribution';
+
+describe('index', () => {
+  it('exports Web Vitals metrics functions', () => {
+    [onCLS, onFCP, onFID, onINP, onLCP, onTTFB].forEach((onFn) =>
+      assert(typeof onFn === 'function')
+    );
+  });
+
+  it('exports Web Vitals metric thresholds', () => {
+    assert.deepEqual(CLSThresholds, [0.1, 0.25]);
+    assert.deepEqual(FCPThresholds, [1800, 3000]);
+    assert.deepEqual(FIDThresholds, [100, 300]);
+    assert.deepEqual(INPThresholds, [200, 500]);
+    assert.deepEqual(LCPThresholds, [2500, 4000]);
+    assert.deepEqual(TTFBThresholds, [800, 1800]);
+  });
+});