MINOR: stats: rename ambiguous stat_l and stat_count

MINOR: stats: rename ambiguous stat_l and stat_count

Statistics were extended with the introduction of stats module. This
mechanism allows to expose various metrics for several haproxy
components. As a consequence of this, some static variables were
transformed to dynamic ones to be able to regroup all statistics
definition.

Rename these variables with more explicit naming :
* stat_lines can be used to generate one line of statistics for any
  module using struct field as value
* metrics and metrics_len are used to stored description of metrics
  indexed by module

Note that info is not integrated in the statistics module mechanism.
However, it could be done in the future to better reflect its purpose.