bench: introduce -min_time argument

When it is not easily possible to stabilize benchmark machine and code
the argument -min_time can be used to specify a minimum duration
that a benchmark should take. E.g. choose -min_time=1000 if you
are willing to wait about 1 second for each benchmark result.

The default is now set to 10ms instead of 0, which should make runs on
fast machines more stable with negligible slowdown.

src/bench/bench.cpp

@@ -61,6 +61,12 @@ void benchmark::BenchRunner::RunAll(const Args& args)
 
         Bench bench;
         bench.name(p.first);
+        if (args.min_time > 0ms) {
+            // convert to nanos before dividing to reduce rounding errors
+            std::chrono::nanoseconds min_time_ns = args.min_time;
+            bench.minEpochTime(min_time_ns / bench.epochs());
+        }
+
         if (args.asymptote.empty()) {
             p.second(bench);
         } else {