[PATCH] Add RetrieveInstrumentation hook for CustomScan providers

Dear PostgreSQL Hackers,

This email proposes a patch to enhance the CustomScan provider interface.
The patch file,
0001-Add-RetrieveInstrumentationCustomScan-hook-for-Custo.patch, is
attached.

*Problem:*

CustomScan providers currently lack a standard method to aggregate
instrumentation data from parallel workers back to the leader process
before the Dynamic Shared Memory (DSM) segment is unlinked. This makes it
difficult to gather comprehensive performance metrics from parallelized
custom scans.

*Solution:*

This patch introduces a new optional hook, RetrieveInstrumentationCustomScan,
to the CustomExecMethods struct. This hook allows custom scan providers to
implement logic to collect and consolidate instrumentation from shared
memory or worker states during the parallel query cleanup phase. This hook
is invoked via the new ExecCustomScanRetrieveInstrumentation function,
called from ExecParallelRetrieveInstrumentation for T_CustomScanState nodes.
Since the hook is optional (checked for NULL before calling), it maintains
full backward compatibility.

*Testing & Compatibility:*

- The patch compiles and passes all core regression tests (make
check-world) on my x86_64 instance.
- The changes are not platform-specific.
- Regression Tests: This patch provides a new *capability* for custom
scan providers. Since the hook's functionality is only realized when
implemented by an extension, specific tests would naturally reside within
that extension rather than in the core regression suite.

This patch does not directly address a specific item on the official TODO
list but enhances the extensibility framework.

I believe this patch is complete and ready for review. I look forward to
any feedback and am happy to make revisions. I will also add this patch to
the next CommitFest.

Thank you,

Siddharth Kothari