Support to drop few metrics(whitelist/blacklist)

[MakarandNsd]

Can we add support to provide a list of metrics which can be whitelisted/blacklisted, so that we need not wait till prometheus drops metrics at relabel stage.

Also a support to add extra labels

[tcolgate]

The current implemention defaults to a straight reverse proxy, which makes this a bit trickier. That said, it isn't a bad idea. I'll leave this open and have a think.