1. Information_schema.innodb_metrics has index_merge counter defined but
   it is not incremented in btr0btr.cc.

2. Users need some other relevant counters like index_page_discards,
   index_page_reorg etc.

So index_page_merge_attempts which replace index_merges is introduced to
distinguish from index_page_merge_successful, also, index_page_reorg_attempts, 
index_page_reorg_successful and index_page_discards are introduced too.

Original patch is provided by Inaam.

Approved by Jimmy, rb#3297