* ESQL: Implement a MetricsAware interface (elastic#120527)

This implements an interface that export the names of the plan nodes and
functions that need to be counted in the metrics.

Also, the metrics are now counted from within the parser. This should
allow correct accounting for the cases where some nodes can appear both
standalone or part other nodes' children (like Aggregate being a child
of INLINESTATS, so no STATS counting should occur).

The functions counting now also validates that behind a name there is
actually a function registered.

Closes elastic#115992.

(cherry picked from commit a4482d4)

* Drop the HashSet gating when counting commands

The telemetry accounting is no longer done in just one place in the parser,
but split, so that no HashSet is required to discard duplicate accounting of
the same node. This lowers the memory requirements.