Add benchmarks

[jupp0r]

This change adds microbenchmarks for various features of the library

• counter
  • incrementing
  • collecting
• gauge
  • incrementing
  • decrementing
  • setting to current time
  • collecting
• histogram
  • observing values for different bucket sizes
  • collecting for differnet bucket sizes
• registry
  • creating families
  • creating metrics inside families with different numbers of label pairs

These benchmarks will come in handy optimizing the library in case any
performance bottlenecks are discovered later.

The benchmarks that matter (incrementing counters, observing histogram
values, ...) show results in the 10s to 100s of nanoseconds, which
looks very good already.

[jupp0r]

bazel run -c opt tests/benchmark/benchmarks
INFO: Found 1 target...
Target //tests/benchmark:benchmarks up-to-date:
  bazel-bin/tests/benchmark/benchmarks
INFO: Elapsed time: 1.682s, Critical Path: 1.56s

INFO: Running command line: bazel-bin/tests/benchmark/benchmarks
Run on (8 X 2300 MHz CPU s)
2016-10-17 15:56:49
Benchmark                              Time           CPU Iterations
--------------------------------------------------------------------
BM_Counter_Increment                  11 ns         11 ns   62947942
BM_Counter_Collect                    84 ns         84 ns    8221752
BM_Gauge_Increment                    11 ns         11 ns   61384663
BM_Gauge_Decrement                    11 ns         11 ns   62148197
BM_Gauge_SetToCurrentTime            199 ns        198 ns    3589670
BM_Gauge_Collect                      86 ns         85 ns    7469136
BM_Histogram_Observe/0               122 ns        122 ns    5839855
BM_Histogram_Observe/1               116 ns        115 ns    5806623
BM_Histogram_Observe/8               126 ns        126 ns    5781588
BM_Histogram_Observe/64              138 ns        138 ns    4895550
BM_Histogram_Observe/512             228 ns        228 ns    2992898
BM_Histogram_Observe/4k              959 ns        958 ns     642231
BM_Histogram_Collect/0               328 ns        327 ns    2002792
BM_Histogram_Collect/1               356 ns        354 ns    1819032
BM_Histogram_Collect/8              1553 ns       1544 ns     454921
BM_Histogram_Collect/64            10389 ns      10287 ns      66759
BM_Histogram_Collect/512           75795 ns      75093 ns       9075
BM_Histogram_Collect/4k           615853 ns     610277 ns       1222
BM_Registry_CreateFamily             195 ns        182 ns    3843894
BM_Registry_CreateCounter/0          319 ns        317 ns    1914132
BM_Registry_CreateCounter/1         2146 ns       2131 ns     408432
BM_Registry_CreateCounter/8         8936 ns       8837 ns      82439
BM_Registry_CreateCounter/64       72589 ns      72010 ns       9248
BM_Registry_CreateCounter/512     694323 ns     686655 ns       1056
BM_Registry_CreateCounter/4k    18246638 ns   18150525 ns         40