message: Add DECLARE_counter/DEFINE_counter for consistency
  Summary: There already existed DECLARE_ and DEFINE_ macros for timeseries and histograms.  Add another set for counters for consistency.
diff: @@ -1102,6 +1102,9 @@ inline fb303::ThreadCachedServiceData& tcData() {
  } // namespace facebook
  +#define DECLARE_counter(varname) \
  + extern ::facebook::fb303::CounterWrapper STATS_##varname
  +
  #define DECLARE_timeseries(varname) \
  extern ::facebook::fb303::TimeseriesWrapper STATS_##varname
  @@ -1114,6 +1117,9 @@ inline fb303::ThreadCachedServiceData& tcData() {
  #define DECLARE_dynamic_histogram(varname, keyNumArgs) \
  extern ::facebook::fb303::DynamicHistogramWrapper<keyNumArgs> STATS_##varname
  +#define DEFINE_counter(varname) \
  + ::facebook::fb303::CounterWrapper STATS_##varname(#varname)
  +
  #define DEFINE_timeseries(varname, ...) \
  ::facebook::fb303::TimeseriesWrapper STATS_##varname(#varname, ##__VA_ARGS__)
  