Add metric to track age of blob queried

[satishkotha]

Summary

Add metric to track age of blob queried

Testing Done

Minor change with existing tests cover functionality

[codecov-commenter]

Codecov Report

❌ Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.78%. Comparing base (52ba813) to head (0806725).
⚠️ Report is 338 commits behind head on master.

Files with missing lines	Patch %	Lines
...java/com/github/ambry/frontend/GetBlobHandler.java	66.66%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3193      +/-   ##
============================================
+ Coverage     64.24%   69.78%   +5.54%     
- Complexity    10398    12807    +2409     
============================================
  Files           840      930      +90     
  Lines         71755    79075    +7320     
  Branches       8611     9458     +847     
============================================
+ Hits          46099    55183    +9084     
+ Misses        23004    20948    -2056     
- Partials       2652     2944     +292     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.