folly: add a simple benchmark for Histogram::addValue()

Summary:
I wrote this an an example for the BENCHMARK_NAMED_PARAM() comments in
D527040, and figured I might as well check it in as a real benchmark.

Test Plan:
Here's the output:

======================================================================
folly/test/HistogramBenchmark.cpp         relative  time/iter  iters/s
======================================================================
addValue(0_to_100)                                    15.43ns   64.81M
addValue(0_to_1000)                                   15.45ns   64.70M
addValue(5k_to_20k)                                   15.46ns   64.67M
======================================================================

Reviewed By: rajat@fb.com

FB internal diff: D552875

folly/test/HistogramBenchmark.cpp

+/*
+ * Copyright 2012 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include "folly/Histogram.h"
+
+#include <gflags/gflags.h>
+
+#include "folly/Benchmark.h"
+#include "folly/Foreach.h"
+
+using folly::Histogram;
+
+void addValue(uint n, int64_t bucketSize, int64_t min, int64_t max) {
+  Histogram<int64_t> hist(bucketSize, min, max);
+  int64_t num = min;
+  FOR_EACH_RANGE (i, 0, n) {
+    hist.addValue(num);
+    ++num;
+    if (num > max) { num = min; }
+  }
+}
+
+BENCHMARK_NAMED_PARAM(addValue, 0_to_100, 1, 0, 100);
+BENCHMARK_NAMED_PARAM(addValue, 0_to_1000, 10, 0, 1000);
+BENCHMARK_NAMED_PARAM(addValue, 5k_to_20k, 250, 5000, 20000);
+
+int main(int argc, char *argv[]) {
+  google::ParseCommandLineFlags(&argc, &argv, true);
+  folly::runBenchmarks();
+  return 0;
+}