ESQL: Improve command resolution in telemetry

[costin]

Description

The ESQL telemetry infrastructure expects each LogicalPlan to map uniquely to a command, (through LogicalPlan#commandName()) an assumption that can't be generalized.
Instead the mapping between the plan and the commands needs to be moved to a separate class / registry / map.
P.S. Furthermore, the telemetry might in time be moved to occur at parsing time to track queries that don't pass the grammar, before the logical AST is fully assembled.

Example: FROM index | INLINESTATS c = count() should count FROM and INLINESTATS but ends up counting FROM, INLINESTATS and STATS (due to the normalized plan).

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[luigidellaquila]

@costin I gave it a try when I implemented APM telemetry, and I ended up using commandName() and making it dynamic (see what we do for METRICS)
Even if we move the logic in a separate class, we'll still have to store some information inside the plan, that is the only thing we have after parsing.

P.S. Furthermore, the telemetry might in time be moved to occur at parsing time to track queries that don't pass the grammar, before the logical AST is fully assembled.

That's the best thing to do for sure, but it's not straight forward. If we have to invest on this thing though, IMHO this is the right approach.