Add basic metrics to example server

[michel-laterman]

Add the following metrics to the example server:

• connections.active.count
• connections.error.count

Metrics will be emitted to stdout if the -emit-metrics flag is used with the example server.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.67%. Comparing base (5574611) to head (0e62b94).
⚠️ Report is 26 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #499   +/-   ##
=======================================
  Coverage   81.67%   81.67%           
=======================================
  Files          27       27           
  Lines        2134     2134           
=======================================
  Hits         1743     1743           
  Misses        266      266           
  Partials      125      125           

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