Exclude so_telegraf from the nightly Postgres backup

pg_dumpall dumped every database, and so_telegraf dominated the result: it
is the only database that grows with grid size and metric volume, while
everything else in the cluster is small and mostly static.

Nothing is lost by skipping it. The data is transient metrics on a 14-day
retention window, roles are globals so the per-minion telegraf logins are
still dumped, and the database itself is rebuilt after a restore without
operator action -- init-db.sh recreates it (run on every highstate by
postgres_bootstrap_soc_db, not just on a fresh volume), telegraf_users.sls
re-provisions the roles and schema, and Telegraf recreates its tables on
first write.

salt/postgres/tools/sbin/so-postgres-backup

@@ -47,7 +47,12 @@ trap 'rm -f "$TMPFILE"' EXIT
 
 # Dump all databases and roles, compress. Write to a temp file so the final
 # filename only ever appears for a complete, verified backup.
-if ! docker exec so-postgres pg_dumpall -U postgres | gzip > "$TMPFILE"; then
+#
+# so_telegraf is excluded: it is transient metrics on a short retention window,
+# it dominates the dump size, and it is rebuilt automatically after a restore --
+# init-db.sh recreates the database and Telegraf recreates its tables on first
+# write. Roles are globals, so the per-minion telegraf logins are still dumped.
+if ! docker exec so-postgres pg_dumpall -U postgres --exclude-database=so_telegraf | gzip > "$TMPFILE"; then
   log "ERROR: pg_dumpall/gzip failed; backup aborted"
   exit 1
 fi