Wrong warning message: "Warning: Cannot compute advanced MySQL metrics"

[LotharSee]

From info page, on a Debian Squeeze:

Warning: Cannot compute advanced MySQL metrics; cannot read mysql pid file /var/run/mysqld/mysqld.pid

It is true that the pid file isn't readable, but the check compute advanced metrics (such as 'mysql.performance.user_time'). It gets the pid with _get_server_pid, using ps.

In fact, the warning command isn't where it should be in the code.
We just have to move it (I will do it), but it is also the good opportunity to refactor this part of the code (to include it in _get_server_pid).