Counters for Index Condition Pushdown

We need counters to track Index Condition Pushdown. In particular, we need to track

• how many times pushed-index-condition check was performed
• how many times the check filtered out a record

(Note: We had also an idea to change counters so that
1. index tuple reads
2. full record reads
are counted separately. Implementation of that task does not kill the need
for counters aimed specifically at index condition pushdown)

Proposed counter names:

Handler_pushed_index_cond_checks N
Handler_pushed_index_cond_filtered N

Implementation to filter a record

• Every storage engine invokes ${engine_name}_idx_cond_func(). That function
  has access to SQL layer, so it should be easy for it to bump up counters.

• Batched Key Access uses a different function. Will need to perform counting there, too.

FYI: MySQL-5.6 has a feature called "InnoDB monitor" which exports there four counters:

select name, count from information_schema.innodb_metrics where name like "icp%";
name count
icp_attempts 1
icp_no_match 0
icp_out_of_range 1
icp_match 0

grep for MONITOR_ICP in the source or look at user interface in:
mysql-test/suite/innodb/t/innodb_monitor.test