Add last_commit_lsn to pg_stat_database

I've previously noted in "Add last commit LSN to
pg_last_committed_xact()" [1] that it's not possible to monitor how
many bytes of WAL behind a logical replication slot is (computing such
is obviously trivial for physical slots) because the slot doesn't need
to replicate beyond the last commit. In some cases it's possible for
the current WAL location to be far beyond the last commit. A few
examples:

- An idle server with checkout_timeout at a lower value (so lots of
empty WAL advance).
- Very large transactions: particularly insidious because committing a
1 GB transaction after a small transaction may show almost zero time
lag even though quite a bit of data needs to be processed and sent
over the wire (so time to replay is significantly different from
current lag).
- A cluster with multiple databases complicates matters further,
because while physical replication is cluster-wide, the LSNs that
matter for logical replication are database specific.

Since we don't expose the most recent commit's LSN there's no way to
say "the WAL is currently 1250, the last commit happened at 1000, the
slot has flushed up to 800, therefore there are at most 200 bytes
replication needs to read through to catch up.

In the aforementioned thread [1] I'd proposed a patch that added a SQL
function pg_last_commit_lsn() to expose the most recent commit's LSN.
Robert Haas didn't think the initial version's modifications to
commit_ts made sense, and a subsequent approach adding the value to
PGPROC didn't have strong objections, from what I can see, but it also
didn't generate any enthusiasm.

As I was thinking about how to improve things, I realized that this
information (since it's for monitoring anyway) fits more naturally
into the stats system. I'd originally thought of exposing it in
pg_stat_wal, but that's per-cluster rather than per-database (indeed,
this is a flaw I hadn't considered in the original patch), so I think
pg_stat_database is the correct location.

I've attached a patch to track the latest commit's LSN in pg_stat_database.

Regards,
James Coleman

1: https://www.postgresql.org/message-id/flat/CAAaqYe9QBiAu+j8rBun_JKBRe-3HeKLUhfVVsYfsxQG0VqLXsA(at)mail(dot)gmail(dot)com