Add default variable value information to performance_schema.variable…

[ejortegau]

This PR addresses the feature request reported under bug #103779. Basically, it allows the client to obtain information about the default value of all variables.

It does so by adding column default_value to performance_schema.variables_info:

mysql> select variable_name, default_value from variables_info where variable_name = 'autocommit';
+---------------+---------------+
| variable_name | default_value |
+---------------+---------------+
| autocommit    | ON            |
+---------------+---------------+
1 row in set (0.00 sec)

mysql> select variable_name, default_value from variables_info where variable_name = 'innodb_buffer_pool_size';
+-------------------------+---------------+
| variable_name           | default_value |
+-------------------------+---------------+
| innodb_buffer_pool_size | 134217728     |
+-------------------------+---------------+
1 row in set (0.01 sec)

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Thanks

[ejortegau]

I am withdrawing the PR, as I realized it is not providing the default value of the variable, but its current value. This adds no value, as this information is already available via perofrmance_schema.global_variables and SHOW GLOBAL VARIABLES.

I may try to fix this in and re-open, or create a new PR, in the future.