[#27387] YSQL: Improve logging for incremental catalog cache refresh

Summary:
This diff makes two changes to improve logging for incremental catalog cache
refresh

(1) added DEBUG in the log text to make its purpose clearer
(2) changed `messages` to `invalidation messages` in the log to improve readability

I tried to also replace YBC_LOG_INFO with elog but that caused some unit tests to fail
because they have `SET client_min_messages TO log;`. One difference between
`YBC_LOG_INFO` and elog is that in case of `YBC_LOG_INFO` the log only goes into the
server log but not to the client.

Because incremental catalog cache refresh is a new feature, these `YBC_LOG_INFO` logging
are used to assist debugging. Once the feature is rolled out and field tested and we are
more confident that it is stable enough, I plan to make them DEBUG log and then replace
`YBC_LOG_INFO` with `elog`.
Jira: DB-16921

Test Plan:
./yb_build.sh release --cxx-test pg_catalog_version-test
Look at the new test logs. E.g.:

```
[ts-2] I0926 18:20:27.510396 1845785 pg_yb_utils.c:3256] YBCommitTransactionContainingDDL: got 26 invalidation messages, local catalog version 2

```

```
[ts-1] I0926 18:20:32.236284 1845757 pg_yb_utils.c:3114] DEBUG: pg null=0, nmsgs=4

```

Reviewers: jason

Reviewed By: jason

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D46997

Author: myang2021

src/postgres/src/backend/utils/misc/pg_yb_utils.c

@@ -3085,7 +3085,8 @@ YBCommitTransactionContainingDDL()
 
 				if (nmsgs > max_allowed)
 				{
-					elog(LOG, "too many messages: %d, max allowed %d", nmsgs, max_allowed);
+					elog(LOG, "too many invalidation messages: %d, max allowed %d",
+						 nmsgs, max_allowed);
 					/*
 					 * If we have too many invalidation messages, write PG null into
 					 * messages so that we fall back to do catalog cache refresh.
@@ -3112,10 +3113,10 @@ YBCommitTransactionContainingDDL()
 			}
 			else
 				Assert(nmsgs == 0);
-			YBC_LOG_INFO("pg null=%d, nmsgs=%d", !currentInvalMessages, nmsgs);
+			YBC_LOG_INFO("DEBUG: pg null=%d, nmsgs=%d", !currentInvalMessages, nmsgs);
 		}
 		else if (ddl_transaction_state.num_committed_pg_txns > 0)
-			YBC_LOG_INFO("num_committed_pg_txns: %d",
+			YBC_LOG_INFO("DEBUG: num_committed_pg_txns: %d",
 						 ddl_transaction_state.num_committed_pg_txns);
 
 		/* Clear yb_sender_pid for unit test to have a stable result. */
@@ -3254,8 +3255,8 @@ YBCommitTransactionContainingDDL()
 	}
 	YBClearDdlHandles();
 	if (increment_done)
-		YBC_LOG_INFO("%s: got %d messages, local catalog version %" PRIu64,
-					 __func__, nmsgs, yb_catalog_cache_version);
+		YBC_LOG_INFO("%s: got %d invalidation messages, local catalog version %" PRIu64,
+			 __func__, nmsgs, yb_catalog_cache_version);
 }
 
 void