Custom or add more property to logger in logRequestCompleted

I've looked at the library and found can't be edit or format custom log in events 'response' when logRequestCompleted: true
log when a request completed

I try to add more field in ```request.raw.res``` but in HAPI docs say "not recommend to do it"
```
request.logger.info(
{
payload: options.logPayload ? request.payload : undefined,
queryParams: options.logQueryParams ? request.query : undefined,
tags: options.logRouteTags ? request.route.settings.tags : undefined,
// note: pino doesnt support unsetting a key, so this next line
// has the effect of setting it or "leaving it as it was" if it was already added via child bindings
req: shouldLogRequestStart(request) ? undefined : request,
res: request.raw.res,
responseTime: (info.completed !== undefined ? info.completed : info.responded) - info.received
myCustomField: myValue
},
'request completed'
)
```
Why I need it: When I using ELK to trace the issue, I need to add x-request-id to know the flow of the request so it makes me terrible. So if I can add more property like errorCode or errorMessage in the only request completed to trace easier

Thank for support.

Start at the library's logRequestCompleted handling and trace how the completed-request log object is assembled. Determine how callers could supply fields such as x-request-id, errorCode, or errorMessage without modifying request.raw.res; done means custom fields appear only in the completed-request log and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.