Add metric to track the response size

[pentreathm]

This metric could be used to diagnose issues with bandwidth, serialization or file transfer.

https://github.com/decentraland/builder-server/blob/b84cb04d0539ef1324893ee16b0d45f455bbc4bd/src/common/ExpressApp.ts#L105-L113

We are currently tracking time but not sizes https://github.com/decentraland/catalyst/blob/7f3cdb6b9c5ce0e628c33f26d32052bda96aa438/commons/servers/metrics.ts#L11