Session statistics, as introduced by 960869da, had several shortcomings:

- an additional GetCurrentTimestamp() call that also impaired the accuracy of
  the data collected

  This can be avoided by passing the current timestamp we already have in
  pgstat_report_stat().

- an additional statistics UDP packet sent every 500ms

  This is solved by adding the new statistics to PgStat_MsgTabstat.
  This is conceptually ugly, because session statistics are not
  table statistics.  But the struct already contains data unrelated
  to tables, so there is not much damage done.

  Connection and disconnection are reported in separate messages, which
  reduces the number of additional messages to two messages per session and a
  slight increase in PgStat_MsgTabstat size (but the same number of table
  stats fit).

- Session time computation could overflow on systems where long is 32 bit.
Reported-By: Andres Freund <andres@anarazel.de>
Author: Andres Freund <andres@anarazel.de>
Author: Laurenz Albe <laurenz.albe@cybertec.at>
Discussion: https://postgr.es/m/20210801205501.nyxzxoelqoo4x2qc%40alap3.anarazel.de
Backpatch: 14-, where the feature was introduced.