LabKit v2

If we ever would release v2 of LabKit, this issue lists what we'd like to fix (i.e. breaking changes).

• tracing/grpc and correlation/grpc both have package name grpccorrelation. !72 (closed)
• Remove deprecated package gitlab.com/gitlab-org/labkit/correlation/raven
• Consider moving metrics/handler* into a sub package, i.e., gitlab.com/gitlab-org/labkit/metrics/http, to avoid name clashes with other "types" of metrics, such as SQL (followup from !90 (comment 446238662)).
• Replace logrus: #24
• Remove global state from the library e.g. reliance on global metrics registry, etc.