MINOR: promex: Add info in the promex context to dump extra counters
The context of the promex applet was extended to support the dump of extra
counters. These counters are not dumped yet, but info to interrupt and
restart the dump are required. The stats module and the relative field
number for this module can now be saved.
In addition support for "extra-counters" parameter was added on the
query-string to dump these counters. Otherwise, no extra-counters are
dumped.