[RFC] *Display* aggregates only.

[LebedevRI]

There is a flag

benchmark/src/benchmark.cc

Lines 75 to 78 in d9cab61

	DEFINE_bool(benchmark_report_aggregates_only, false,
	"Report the result of each benchmark repetitions. When 'true' is "
	"specified only the mean, standard deviation, and other statistics "
	"are reported for repeated benchmarks.");

and a call

benchmark/include/benchmark/benchmark.h

Lines 837 to 840 in d9cab61

	// Specify if each repetition of the benchmark should be reported separately
	// or if only the final statistics should be reported. If the benchmark
	// is not repeated then the single result is always reported.
	Benchmark* ReportAggregatesOnly(bool value = true);

But that affects everything, every reporter, destination:

benchmark/src/benchmark.cc

Line 316 in d9cab61

if (report_aggregates_only) reports.clear();

It would be quite useful to have an ability to be more picky.

More specifically, i would like to be able to only see the aggregates in the on-screen output,
but for the file output to still contain everything. The former is useful in case of a lot of repetition
(or even more so if every iteration is reported separately), while the former is great for tooling.

Now the problem. I'm not sure how best to do it.
The most straight-forward solution i can think of is to filter the results here:

benchmark/src/benchmark.cc

Lines 466 to 471 in d9cab61

	std::vector<BenchmarkReporter::Run> reports =
	RunBenchmark(benchmark, &complexity_reports);
	display_reporter->ReportRuns(reports);
	if (file_reporter) file_reporter->ReportRuns(reports);
	flushStreams(display_reporter);
	flushStreams(file_reporter);

So the file reporter will still receive the full std::vector<BenchmarkReporter::Run> reports, while
display_reporter may (depending on the configuration) only receive the aggregates.

Alternative solutions could include modifying the API of BenchmarkReporter,
and every implementation, to skip non-aggregates, depending on some additional config params.

Thoughts?