add express-session instrumentation

[simon-id]

What does this PR do?

Motivation

Plugin Checklist

• Unit tests.
• TypeScript definitions.
• TypeScript tests.
• API documentation.
• CircleCI jobs/workflows.
• Plugin is exported.

Additional Notes

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 70.79%. Comparing base (4d6a8e3) to head (2e652bb).
Report is 46 commits behind head on master.

Files with missing lines	Patch %	Lines
packages/dd-trace/src/appsec/index.js	25.00%	6 Missing ⚠️
packages/dd-trace/src/appsec/sdk/set_user.js	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5060      +/-   ##
==========================================
- Coverage   76.22%   70.79%   -5.44%     
==========================================
  Files         291      154     -137     
  Lines       13607     5632    -7975     
==========================================
- Hits        10372     3987    -6385     
+ Misses       3235     1645    -1590     

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