MEDIUM: trace: create a new "trace" statement in the "global" section

MEDIUM: trace: create a new "trace" statement in the "global" section

The exact same commands as those from the CLI may be pre-loaded at boot
time by passing them one per line after the "trace" keyword in the global
section; i.e. just copy-pasting all commands directly there will do the
job. Note that if a ring is mentioned, it needs to be declared before the
global section. Another option is to append another global section after
"ring".

For now the keyword is marked as experimental to discourage its broad
adoption by default. "expose-experimental-directives" needs to be placed
in the global section to expose it.