[APPSEC-11052] Fix a bug with ASM span tags reporting the number of WAF failed loaded rules

[GustavoCaso]

What does this PR do?

Fix a bug introduced on #3087

Reported the loaded rules count on the _dd.appsec.event_rules.error_count tag.

I also, added a new system test scenario on our GH actions to avoid introducing a regression again.

Motivation:

Additional Notes:

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[ivoanjo]

👍 LGTM

[codecov-commenter]

Codecov Report

Merging #3106 (43d2564) into master (f2c8241) will decrease coverage by 0.01%.
Report is 14 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3106      +/-   ##
==========================================
- Coverage   98.16%   98.15%   -0.01%     
==========================================
  Files        1323     1323              
  Lines       75155    75155              
  Branches     3428     3428              
==========================================
- Hits        73773    73772       -1     
- Misses       1382     1383       +1     

Files Changed	Coverage Δ
.../datadog/appsec/contrib/rack/request_middleware.rb	95.55% <100.00%> (ø)

... and 1 file with indirect coverage changes

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