Improve Prometheus integration

• Statistics: Integrate a Gauge and a Histogram.
• LocalFile: Use pwrite/pread to make reading and writing lock-less.
• Renamed: benchmark to benchmark_io.
• Integrate benchmark for shuffle_read and shuffle_write.
• Use preallocated buffers in the TCP transport.
• Remove grpchelloworld.
• HTTPServer: Add a workaround to support multiple processes.
• Fix the ports.
• Avoid extra copies in the concurrent map.