Add memory_limit_hits to pg_stat_replication_slots

Hi hackers,

I think that it's currently not always possible to determine how many times
logical_decoding_work_mem has been reached.

For example, say a transaction is made of 40 subtransactions, and I get:

  slot_name   | spill_txns | spill_count | total_txns
--------------+------------+-------------+------------
 logical_slot |         40 |          41 |          1
(1 row)

Then I know that logical_decoding_work_mem has been reached one time (total_txns).

But as soon as another transaction is decoded (that does not involve spilling):

  slot_name   | spill_txns | spill_count | total_txns
--------------+------------+-------------+------------
 logical_slot |         40 |          41 |          2
(1 row)

Then we don't know if logical_decoding_work_mem has been reached one or two 
times.

Please find attached a patch to $SUBJECT, to report the number of times the 
logical_decoding_work_mem has been reached.

With such a counter one could get a ratio like total_txns/memory_limit_hits.

That could help to see if reaching logical_decoding_work_mem is rare or 
frequent enough. If frequent, then maybe there is a need to adjust
logical_decoding_work_mem.

Based on my simple example above, one could say that it might be possible to get
the same with:

(spill_count - spill_txns) + (stream_count - stream_txns)

but that doesn't appear to be the case with a more complicated example (277 vs 247):

  slot_name   | spill_txns | spill_count | total_txns | stream_txns | stream_count | memory_limit_hits |
(spc-spct)+(strc-strt)

--------------+------------+-------------+------------+-------------+--------------+-------------------+------------------------
 logical_slot |        405 |         552 |         19 |           5 |          105 |               277 |
   247
 
(1 row)

Not sure I like memory_limit_hits that much, maybe work_mem_exceeded is better?

Looking forward to your feedback,

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com