Add remote_ip to Labkit::Context.KNOWN_KEYS

This is in support of gitlab-org/gitlab#232645 (closed).

While in most cases remote_ip will be propagated via X-Forwarded-For header, it may be useful to propagate it through the context in other cases, in particular GRPC (gitaly).

The longer-term plan is to unify all of this, see gitlab-org/labkit!93. But in the mean time, we can still get some value by passing the information to gitaly directly.