consider including server_version in explain(settings)

[Justin Pryzby]

explain(SETTINGS) was implemented to show relevant settings for which an odd
value could affect a query but could be forgotten during troubleshooting.

This is a "concept" patch to show the version, which is frequently requested on
-performance list and other support requests.  If someone sends
explain(settings), they don't need to also (remember to) send the version..

postgres=# explain(settings)SELECT;
 Result  (cost=0.00..0.01 rows=1 width=0)
 Settings: server_version_num = '130000', work_mem = '128MB'

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 85ca2b3..2edc83c 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -3143,7 +3143,7 @@ static struct config_int ConfigureNamesInt[] =
                {"server_version_num", PGC_INTERNAL, PRESET_OPTIONS,
                        gettext_noop("Shows the server version as an integer."),
                        NULL,
-                       GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+                       GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_EXPLAIN
                },
                &server_version_num,
                PG_VERSION_NUM, PG_VERSION_NUM, PG_VERSION_NUM,
@@ -8955,7 +8955,7 @@ get_explain_guc_options(int *num)
                }
 
                /* skip GUC variables that match the built-in default */
-               if (!modified)
+               if (!modified && strcmp(conf->name, "server_version_num"))
                        continue;
 
                /* assign to the values array */