track generic and custom plans in pg_stat_statements

Hi,

Currently, there is little visibility for queries that are being executed
using generic or custom plans. There is pg_prepared_statements which
show generic_plans and custom_plans as of d05b172a760, but this
information is backend local and not very useful to a DBA that wishes
to track this information cumulatively and over time. [0] had intentions
to add these counters to pg_stat_statements, but was withdrawn due
to timing with the commitfest at the time [0] and never picked back up again.

I think it's useful to add these counters.

Therefore, the attached patch adds two new columns to pg_stat_statements:
"generic_plan_calls" and "custom_plan_calls". These columns track the
number of executions performed using a generic or custom plan.

Only non-utility statements are counted, as utility statements with an
optimizable parameterized query (i.e. CTAS) cannot be called with PREPARE.
Additionally, when such statements are repeatedly executed using an extended
protocol prepared statement, pg_stat_statements may not handle them properly,
since queryId is set to 0 during pgss_ProcessUtility.

To avoid introducing two additional counters in CachedPlan, the existing
boolean is_reusable—which determines whether a generic plan is reused to
manage lock requirements—has been repurposed as an enum. This enum now
tracks different plan states, including "generic reused", "generic first time"
and "custom". pg_stat_statements uses these states to differentiate between
generic and custom plans for tracking purposes. ( I felt this was better than
having to carry 2 extra booleans in CachedPlan for this purpose, but that will
be the alternative approach ).

Not included in this patch and maybe for follow-up work, is this information
can be added to EXPLAIN output and perhaps pg_stat_database. Maybe that's
a good idea also?

This patch bumps the version pf pg_stat_statements.

--
Sami Imseih
Amazon Web Services (AWS)

[0] https://www.postgresql.org/message-id/add1e591fbe8874107e75d04328859ec%40oss.nttdata.com