feat(volo-http): add CommonStats and ServerStats struct to record lifecycle timings

[Joshuahoky]

Motivation

Volo-http currently lacks visibility into key HTTP lifecycle timings, such as:

• Time to read request headers and body
• Time spent handling the request
• Time to write the response body

Solution

Added ServerStats in volo-http/src/context/server.rs to store timestamps for:

• request header read start/finish
• request body read start/finish
• handler execution start/finish
• response body write start/finish

Also instrumented the request handling flow to populate the ServerHandleStart / Finish fields of ServerStats

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.84%. Comparing base (c971091) to head (efdf473).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #649   +/-   ##
=======================================
  Coverage   44.84%   44.84%           
=======================================
  Files         162      162           
  Lines       20078    20081    +3     
=======================================
+ Hits         9003     9006    +3     
  Misses      11075    11075           

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