perf list: Support printing MetricExpr with --debug

perf list: Support printing MetricExpr with --debug

Output the metric expr in perf list when --debug is specified, so that
the user can check the formula.

Before:

  % perf list
    ...
    unc_m_power_channel_ppd
         [Cycles where DRAM ranks are in power down (CKE) mode. Derived from unc_m_power_channel_ppd. Unit:
          uncore_imc]
          uncore_imc_2/event=0x85/

After:

  % perf list --debug
    ...
    unc_m_power_channel_ppd
         [Cycles where DRAM ranks are in power down (CKE) mode. Derived from unc_m_power_channel_ppd. Unit:
          uncore_imc]
          Perf: uncore_imc_2/event=0x85/ MetricExpr: (unc_m_power_channel_ppd / unc_m_clockticks) * 100.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170320201711.14142-12-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>