pgsql: Improve check for detection of pending data in backend statistic

Improve check for detection of pending data in backend statistics

The callback pgstat_backend_have_pending_cb() is used as a way for
pg_stat_report() to detect if there is any pending data for backend
statistics.

It did not include a check based on pgstat_tracks_backend_bktype(), that
discards processes whose backend types do not support backend
statistics. The logic is not a problem on HEAD, as processes that do
not support backend statistics cannot touch PendingBackendStats, so the
callback would always report that there is no pending data in this case.
However, we would run into trouble once backend statistics include
portions of pending stats that are not always zeroed, like pgWalUsage.

There is no reason for pgstat_backend_have_pending_cb() to not check
for pgstat_tracks_backend_bktype(), anyway, and this pattern is safer in
the long run, so let's update the code to do so.

While on it, this commit adds a proper initialization to
PendingBackendStats.

Author: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Co-authored-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/Z8l6EMM4ImVoWRkg@ip-10-97-1-34.eu-west-3.compute.internal

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9a8dd2c5a6d9d2538444c156e6b273b1b4f4e3a4

Modified Files
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src/backend/utils/activity/pgstat_backend.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)