perf stat: Increase perf metric output resolution

Add another digit of precision to the perf metrics output.

Before:

  $ /usr/bin/perf stat --metrics run_cpi /bin/ls
  [...]
           4,345,526      pm_run_cyc                #      1.1 run_cpi
           3,818,069      pm_run_inst_cmpl
  [...]
  $ /usr/bin/perf stat --metrics run_cpi --metric-only /bin/ls
  [...]
               run_cpi
                   1.1
  [...]

After:

  $ perf stat --metrics run_cpi /bin/ls
  [...]
           4,280,882      pm_run_cyc                #     1.12 run_cpi
           3,817,016      pm_run_inst_cmpl
  [...]
  $ perf stat --metrics run_cpi --metric-only /bin/ls
  [...]
               run_cpi
                  1.06
  [...]

Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Paul Clarke <pc@us.ibm.com>
LPU-Reference: 1588861087-31280-1-git-send-email-pc@us.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>