flow.perf: Add auto-timer around existing Clock_types et al: a-la boost.timer.

[ygoldfeld]

Filed by @ygoldfeld pre-open-source:

An easy utility for casual timing (conceptually just like https://www.boost.org/doc/libs/1_77_0/libs/timer/doc/index.html):

{
  flow::perf::Auto_timer t(logger, flow::log::Sev::S_INFO, "op name", <a Clock_types_subset>);
  // ...do stuff, possibly break out of {} early, whatever...
}
// When block exits, t dtor runs and logs, in the Checkpointing_timer style, the total duration results. Use minimal timing to cut overhead completely: Checkpointing_timer::now() 2x. No need to use a Checkpointing_timer object since always just 1 checkpoint.

Could also use it in Flow-IPC's perf_demo if useful there.

[ygoldfeld]

Note - boost.timer (linked in description) may well be sufficient and has its own knobs and whatever. This one has synergy with flow.perf and flow.log, basically rounding out its capabilities with Checkpointing_timer machinery at the core (and logging via flow.log). A value-add if you will.