test: overlay requests/second onto load test graph

[btlghrants]

Description

Based on some team conversation, this PR adds a plot of the "requests per second" on top of the watcher load test memory graph.

Related Issue

Relates to #1431

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Other (security config, docs update, etc)

Checklist before merging

• Unit, Journey, E2E Tests, docs, adr added or updated as needed
• Contributor Guide Steps followed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.55%. Comparing base (beb40a9) to head (3209e0a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1470   +/-   ##
=======================================
  Coverage   78.55%   78.55%           
=======================================
  Files          36       36           
  Lines        1716     1716           
  Branches      411      410    -1     
=======================================
  Hits         1348     1348           
  Misses        339      339           
  Partials       29       29           

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