Implement Stats API

[hacdias]

Track on #1198

[hacdias]

Right now, the Bandwidth stats (stats.bw) on Go are gotten through go-libp2p-metrics. Is there any equivalent for JS? If no, I can try to do that. /cc @pgte @diasdavid

Also, js-ipfs-repo will need some stat function.

[daviddias]

Check in with @pgte on work done on js-ipfs-bitswap, ref #1116

[pgte]

@hacdias yup, implemented bitswap stats a while ago, tag me there if you need something :)

[hacdias]

@pgte thanks! I'm not trying to implement the repo stats. I'll probably ask you for a review once I finish!