API Feature Flags Integration

You are a platform engineer specializing in feature management and progressive delivery.

## CONTEXT
I need to implement feature flags in my API.

**Requirements:**
- Flag Types: {{FLAG_TYPES}}
- Targeting Rules: {{TARGETING}}
- Integration: {{INTEGRATION}}
- Monitoring: {{MONITORING}}

## TASK
Design feature flag integration:

### 1. FLAG DEFINITION
```json
{
  "key": "new_checkout_flow",
  "type": "boolean",
  "defaultValue": false,
  "targeting": {
    "rules": [
      {
        "conditions": [{ "attribute": "plan", "operator": "eq", "value": "premium" }],
        "value": true
      }
    ],
    "percentageRollout": 20
  }
}
```

### 2. API INTEGRATION
```typescript
async function handler(req, res) {
  const flags = await getFlags(req.user);

  if (flags.isEnabled('new_checkout_flow')) {
    return newCheckoutFlow(req, res);
  }
  return legacyCheckoutFlow(req, res);
}
```

### 3. CLIENT EXPOSURE
```json
GET /api/features
{
  "features": {
    "new_checkout_flow": true,
    "dark_mode": false,
    "beta_features": ["feature_a", "feature_b"]
  }
}
```

### 4. TARGETING STRATEGIES
- User attributes
- Percentage rollout
- Geographic targeting
- Time-based activation

### 5. A/B TESTING
- Variant assignment
- Metrics tracking
- Statistical significance

### 6. KILL SWITCHES
Emergency feature disable mechanism.