[Instrumentation.AspNet] Fix metric http.server.duration always being zero

[qhris]

Fixes a bug introduced in #1388 as mentioned by @vishweshbankwar.

Changes

• Explicitly set the end duration of the activity before onRequestStoppedCallback is called.
• Updated tests to catch this in the future.

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes

[codecov]

Codecov Report

Merging #1425 (4947b6d) into main (71655ce) will decrease coverage by 2.80%.
Report is 48 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1425      +/-   ##
==========================================
- Coverage   73.91%   71.11%   -2.80%     
==========================================
  Files         267       13     -254     
  Lines        9615      277    -9338     
==========================================
- Hits         7107      197    -6910     
+ Misses       2508       80    -2428     

Flag	Coverage Δ
unittests-Instrumentation.AspNet	71.11% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...ation.AspNet.TelemetryHttpModule/ActivityHelper.cs	85.71% <100.00%> (+0.46%)	⬆️

... and 250 files with indirect coverage changes