Hardcoded slot_name in pg_stat_wal_receiver collector

[maxout123]

https://github.com/prometheus-community/postgres_exporter/blob/b126e621db083d4299120c02b157e0e13aafa7a4/collector/pg_stat_walreceiver.go#L118C3-L118C68

slot_name format is hardcoded to be repmgr_slot_([0-9]*), while slots can have any name you like.
With custom slot names the whole collector doesn't work with collector=stat_wal_receiver msg="Skipping wal receiver stats because upstream_node is null" message