[APPSEC-56187] Rework WAF span metrics collection

[Strech]

What does this PR do?

Encapsulates metrics collection into AppSec::Context which exposes public API to call security checks. And restructure some other classes responsibilities.

Motivation:

The end goal is to reduce abstraction leak from instrumentation and accumulate it in a single place for the future refactoring.

⚠️ This is not the end-goal, but rather mid-state.

Change log entry

No.

Additional Notes:

In a nutshell

1. AppSec::Processor::Context moved into AppSec::SecurityEngine::Runner and lost some responsibilities it should not has.
2. A new self-sufficient representation of the WAF run introduced AppSec::SecurityEngine::Result::{Ok, Match, Error} with united interface and stats.
3. Security events storage moved into Context too to remove the knowledge leak from security engine side.
4. And AppSec module is now responsible for knowing is API-Security enabled or not.

How to test the change?

If the CI and System-Tests are passing - we are good to go.

[datadog-datadog-prod-us1]

Datadog Report

Branch report: appsec-56187-move-waf-span-metrics-into-context
Commit report: b6e2f20
Test service: dd-trace-rb

✅ 0 Failed, 22063 Passed, 1477 Skipped, 5m 10.33s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-15 18:21:08

Comparing candidate commit b6e2f20 in PR branch appsec-56187-move-waf-span-metrics-into-context with baseline commit 9e3a3e3 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 31 metrics, 2 unstable metrics.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.84946% with 8 lines in your changes missing coverage. Please review.

Project coverage is 97.71%. Comparing base (9e3a3e3) to head (b6e2f20).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/datadog/appsec/contrib/rack/gateway/watcher.rb	50.00%	3 Missing ⚠️
.../datadog/appsec/contrib/sinatra/gateway/watcher.rb	50.00%	2 Missing ⚠️
...ib/datadog/appsec/contrib/rails/gateway/watcher.rb	50.00%	1 Missing ⚠️
lib/datadog/appsec/monitor/gateway/watcher.rb	50.00%	1 Missing ⚠️
lib/datadog/appsec/security_engine/runner.rb	97.95%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4291   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files        1356     1358    +2     
  Lines       82489    82498    +9     
  Branches     4219     4221    +2     
=======================================
+ Hits        80604    80615   +11     
+ Misses       1885     1883    -2     

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