This revert 17d807f0 and re-adds the metadata plugin as a plugin that
just sets a label to a value function.
Add package documentation on how to use the metadata package. Make it
clear that any caching is up to the Func implemented.
There are now - no in tree users. We could add the request metadata by
default under names that copy request.Request, i.e
request/ip - remote IP
request/port - remote port
Variables.go has been deleted.
Signed-off-by: Miek Gieben <miek@miek.nl>
99800a687c