Add support for pg_stat_wal

Use case. Why is this important?
I have a use case from my experience. I had huge spikes on checkpoints. At first, I thought the checkpointer was late and that I needed to increase max_wal_size/checkpoint_timeout, but it didn't help. I accidentally noticed that wal_fpi increased along with the spikes. To prove this, my workaround was to sample it with an SQL exporter, and it turned out to be true. The core problem was that we had a table with a GUID index and a huge insert workload, which rapidly dirties almost all the index's pages, triggering massive wal_fpi on checkpoints. Having at least wal_fpi would be very useful to quickly discover such problems. Also, I think it would be useful to have the entire pg_stat_wal in the exporter, but for other columns, I don't have such motivation.