Get memory contexts of an arbitrary backend process

Hi,

After commit 3e98c0bafb28de, we can display the usage of the
memory contexts using pg_backend_memory_contexts system
view.

However, its target is limited to the process attached to
the current session.

As discussed in the thread[1], it'll be useful to make it
possible to get the memory contexts of an arbitrary backend
process.

Attached PoC patch makes pg_get_backend_memory_contexts()
display meory contexts of the specified PID of the process.

=# -- PID of the target process is 17051
=# SELECT * FROM pg_get_backend_memory_contexts(17051) ;
name | ident | parent | level |
total_bytes | total_nblocks | free_bytes | free_chunks | used_bytes

-----------------------+-------+------------------+-------+-------------+---------------+------------+-------------+------------
TopMemoryContext | | | 0 |
68720 | 5 | 16816 | 16 | 51904
RowDescriptionContext | | TopMemoryContext | 1 |
8192 | 1 | 6880 | 0 | 1312
MessageContext | | TopMemoryContext | 1 |
65536 | 4 | 19912 | 1 | 45624
...

It doesn't display contexts of all the backends but only
the contexts of specified process.
I think it would be enough because I suppose this function
is used after investigations using ps command or other OS
level utilities.

The rough idea of implementation is like below:

1. send a signal to the specified process
2. signaled process dumps its memory contexts to a file
3. read the dumped file and display it to the user

Any thoughts?

[1]
https://www.postgresql.org/message-id/72a656e0f71d0860161e0b3f67e4d771%40oss.nttdata.com

Regards,

--
Atsushi Torikoshi
NTT DATA CORPORATION