Add a MultiProvider implementation for metrics/multi package

Currently there is no implementation of a [Provider](https://github.com/go-kit/kit/blob/master/metrics/provider/provider.go) interface for the [metrics/multi](https://godoc.org/github.com/go-kit/kit/metrics/multi) package. Adding a `MultiProvider` that accepts a slice of other `Provider` implementations would make using `multi` package more convenient.