PG-1621: fix cmd_type mostly showing 0 values (#538)

This was actually caused by two bugs internally:

* cmd_type was only set in some codepaths, other parts of the code
never set a value. Depending on which query / how was executed,
it was possibly never changed (after a reset to 0)
* the update first set the cmd_type, then reset all counters. As
the cmd_type is stored within the counters for some reason, this
reset its value to 0 in most execution paths, even if it was corretly
set before.

And according to this the fix is simple:

* cmd_type is now set in all codepaths except for failing queries,
as we only have the error string in this case, without the type.
* in the update logic, we again overwrite cmd_type with the proper
value after a reset

regression/expected/version.out

@@ -2,7 +2,7 @@ CREATE EXTENSION pg_stat_monitor;
 SELECT pg_stat_monitor_version();
  pg_stat_monitor_version 
 -------------------------
- 2.1.1
+ 2.2.0
 (1 row)
 
 DROP EXTENSION pg_stat_monitor;