Benchmarks for string atom field transformations

- Added benchmarks for trimming trailing numeric characters via:
  1. Targeting trailing numbers using string::find_last_not_of
  2. Targeting trailing numbers using std::is_digit
  3. std::regex_replace
  4. POSIX C regex

Bug: 307768377
Test: atest statsd_benchmark
Change-Id: Ib0665f590e3125f0a9400cef584d1afdb108b797
2 files changed