time_stats.md

1. remove all ad-hoc classes: Remove APIServingTimingMixin, RequestTimingMetricsMixin, TimeStats. Unify to the new API.
   • e.g. many time staus related in ReqState: finished_time
2. remove ad-hoc code in the critical path. e.g.,

                # Record response sent time right before we send response.
              if not state.response_sent_to_client_ts:
                  state.response_sent_to_client_ts = time.time()
                  out["meta_info"][
                      "response_sent_to_client_ts"
                  ] = state.response_sent_to_client_ts

3. all api comes from a single base class. ReqTimeStatsBase
4. The data structure only records a list of monotonically increasing timestamps. People can do analysis (substraction) later.
5. How to deal with _calculate_timing_metrics. Move to the new dedicated file.

Success metrics:

1. If we search for time.time() or time.perf_counter(). They only appear in a new file.