Don't enforce logged keys to be known by Labkit

Discussed in gitlab-com/gl-infra/scalability#128 (comment 462598081) Also useful for gitlab-org/gitlab!75542 (merged)

It would be handy if all projects used the same keys to log data, which is why I initially added this list, but having to modify all of the projects each time would become cumbersome to keep it in sync will become cumbersome.

So this changes that all keys are allowed: when passing them in through Labkit, a meta. prefix is added (for logs). These will be propagated by Labkit when found in Sidekiq jobs.

Now we need to pay attention in the other projects that will push into a context that they use the same format. But having duplicate information logged in different fields is likely better than losing information.