Add pg_stat_database counters for sessions and session time

This add counters for number of sessions, the different kind of session
termination types, and timers for how much time is spent in active vs
idle in a database to pg_stat_database.

Internally this also renames the parameter "force" to disconnect. This
was the only use-case for the parameter before, so repurposing it to
this mroe narrow usecase makes things cleaner than inventing something
new.

Author: Laurenz Albe
Reviewed-By: Magnus Hagander, Soumyadeep Chakraborty, Masahiro Ikeda
Discussion: https://postgr.es/m/b07e1f9953701b90c66ed368656f2aef40cac4fb.camel@cybertec.at

src/backend/utils/error/elog.c

@@ -72,6 +72,7 @@
 #include "libpq/pqformat.h"
 #include "mb/pg_wchar.h"
 #include "miscadmin.h"
+#include "pgstat.h"
 #include "postmaster/bgworker.h"
 #include "postmaster/postmaster.h"
 #include "postmaster/syslogger.h"
@@ -656,6 +657,13 @@ errfinish(const char *filename, int lineno, const char *funcname)
 		fflush(stdout);
 		fflush(stderr);
 
+		/*
+		 * Let the statistics collector know. Only mark the session as
+		 * terminated by fatal error if there is no other known cause.
+		 */
+		if (pgStatSessionEndCause == DISCONNECT_NORMAL)
+			pgStatSessionEndCause = DISCONNECT_FATAL;
+
 		/*
 		 * Do normal process-exit cleanup, then return exit code 1 to indicate
 		 * FATAL termination.  The postmaster may or may not consider this