`documentdb_bg_worker_database_name` parameter uses prefix incorrectly

[AlekSi]

Describe the bug

documentdb_bg_worker_database_name should be documentdb.bg_worker_database_name, with a dot instead of an underscore. Using underscore makes changing this parameter impossible because of the reserved prefix.

documentdb/pg_documentdb/src/configs/background_job_configs.c

Lines 159 to 179 in 4125c4b

	DefineCustomStringVariable(
	psprintf("%s_bg_worker_database_name", prefix),
	gettext_noop("Database to which background worker will connect."),
	NULL,
	&BackgroundWorkerDatabaseName,
	DEFAULT_BG_DATABASE_NAME,
	PGC_POSTMASTER,
	GUC_SUPERUSER_ONLY,
	NULL, NULL, NULL);
	DefineCustomIntVariable(
	psprintf("%s.bg_worker_latch_timeout", prefix),
	gettext_noop("Latch timeout inside main thread of bg worker leader."),
	NULL,
	&LatchTimeOutSec,
	DEFAULT_BG_LATCH_TIMEOUT_SEC,
	0,
	200,
	PGC_POSTMASTER,
	GUC_SUPERUSER_ONLY,
	NULL, NULL, NULL);

Note how the first parameter uses an underscore while the second (and all others) use a dot.

documentdb/pg_documentdb/src/pg_documentdb.c

Lines 50 to 51 in 4125c4b

	MarkGUCPrefixReserved("documentdb");
	InitializeDocumentDBBackgroundWorker("pg_documentdb", "documentdb", "documentdb");

Environment

Any.

Reproduction Steps

1. Try to set documentdb_bg_worker_database_name in a configuration file.
2. Get unrecognized configuration parameter "documentdb_bg_worker_database_name" error.

Expected behavior

It is possible to change the database name.

Actual behavior

It is not possible to change the database name.

Additional context

FerretDB/FerretDB#5278