Folly/Benchmark: Make user counters readable

Summary:
When a user counter is added to benchmarks, the counter's value is written as a bare integer.
This change gives chance to user to either still keep it reported as bare integer, a time value or a metric value.

Differential Revision: D14451341

fbshipit-source-id: 95e64a89c201bb385d9a403f3442fa70c2ac7e2a

folly/Benchmark.cpp

@@ -322,7 +322,20 @@ class BenchmarkResultsPrinter {
       }
       for (auto const& name : counterNames_) {
         if (auto ptr = folly::get_ptr(datum.counters, name)) {
-          printf("  %-*d", int(name.length()), *ptr);
+          switch (ptr->type) {
+            case UserMetric::TIME:
+              printf(
+                  "  %-*s", int(name.length()), readableTime(*ptr, 2).c_str());
+              break;
+            case UserMetric::METRIC:
+              printf(
+                  "  %-*s",
+                  int(name.length()),
+                  metricReadable(*ptr, 2).c_str());
+              break;
+            default:
+              printf("  %-*d", int(name.length()), (int)*ptr);
+          }
         } else {
           printf("  %-*s", int(name.length()), "NaN");
         }

folly/Benchmark.h

@@ -53,7 +53,23 @@ inline bool runBenchmarksOnFlag() {
   return FLAGS_benchmark;
 }
 
-using UserCounters = std::unordered_map<std::string, int>;
+struct UserMetric {
+  enum Type { CUSTOM, TIME, METRIC };
+  int value;
+  Type type{CUSTOM};
+  UserMetric() = default;
+  template <typename T>
+  /* implicit */ UserMetric(T val, Type typ = CUSTOM)
+      : value(static_cast<int>(val)), type(typ) {}
+  operator int() const {
+    return value;
+  }
+  operator int&() {
+    return value;
+  }
+};
+
+using UserCounters = std::unordered_map<std::string, UserMetric>;
 
 namespace detail {
 struct TimeIterData {