In `pgStatWalReceiverQueryTemplate`, the order of the columns (when `hasFlushedLSN == true`) is:
- ...
- `receive_start_lsn`
- `flushed_lsn`
- `receive_start_tli`
- ...
However, columns were scanned in this order:
- ...
- `receive_start_lsn` -> `receiveStartLsn`
- `receive_start_tli` -> `flushedLsn` (!)
- `flushed_lsn` -> `receiveStartTli` (!)
- ...
This incorrect hydration of variables also manifests as swapped values for the
`pg_stat_wal_receiver_flushed_lsn` and `pg_stat_wal_receiver_receive_start_tli` metrics.
This seems to be a bug that has existed since the initial implementation:
- 2d7e152751
- https://github.com/prometheus-community/postgres_exporter/pull/844
In this patch, I'm:
- fixing the `.Scan()`, so that it hydrates variables in the correct order
- adjusting the order in which metrics are pushed out to the channel,
to follow the order we consume them in
(.., `receive_start_lsn`, `flushed_lsn`, `receive_start_tli`, ..)
- adjusting the walreceiver tests, to follow the new order (which matches .`Scan()`)
- fixing a small identation issue in `pgStatWalReceiverQueryTemplate`
Signed-off-by: Slavi Pantaleev <slavi@devture.com>
