quests: QuestMetrics helpers (purchaseOffer/purchaseItem), README quests section

src/index.ts

@@ -27,6 +27,7 @@ export type {
 export type { LeaderboardsService, LeaderboardHandle } from './leaderboards/LeaderboardsService.js';
 export type { BattlePassService } from './battlepass/BattlePassService.js';
 export type { QuestsService } from './quests/QuestsService.js';
+export * as QuestMetrics from './quests/QuestMetrics.js';
 
 // Observable state primitives
 export { SyncedState } from './state/SyncedState.js';