Fixes: #13512
Defined and exported type for `metric` used in [reportWebVitals](https://nextjs.org/docs/advanced-features/measuring-performance)
```
export function reportWebVitals(metric) {
if (metric.label === 'custom') {
console.log(metric) // The metric object ({ id, name, startTime, value, label }) is logged to the console
}
}
```
One can now do
```
import { NextWebVitalsMetric } from 'next/app'
export function reportWebVitals(metric: NextWebVitalsMetric ) {
if (metric.label === 'custom') {
console.log(metric) // The metric object ({ id, name, startTime, value, label }) is logged to the console
}
}
```
