perf parse-events: Add new "metric-id" term

Add a new "metric-id" term to events so that metric parsing can set an
ID that can be reliably looked up.

Metric parsing currently will turn a metric like "instructions/cycles"
into a parse events string of "{instructions,cycles}:W".

However, parse-events may change "instructions" into "instructions:u" if
perf_event_paranoid=2.

When this happens expr__resolve_id currently fails as stat-shadow adds
the ID "instructions:u" to match with the counter value and the metric
tries to look up the ID just "instructions".

A later patch will use the new term.

An example of the current problem:

  $ echo -1 > /proc/sys/kernel/perf_event_paranoid
  $ perf stat -M IPC /bin/true
   Performance counter stats for '/bin/true':

           1,217,161      inst_retired.any          #     0.97 IPC
           1,250,389      cpu_clk_unhalted.thread

         0.002064773 seconds time elapsed

         0.002378000 seconds user
         0.000000000 seconds sys

  $ echo 2 > /proc/sys/...