Make reactor-core / reactive-streams context propagation Context-first

[amarziali]

What Does This Do

Reworks the reactor-core-3.1 and reactive-streams-1.0 instrumentations to propagate the full Context (instead of just AgentSpan). The new behaviour refactor only a bit the code and should not add major changes.
The context is only propagated on the reactor chain - the user can only infer a SpanContext but not a full one since Context is not part of the public API.
Also, we used instanceOf to identify Reactor's context-aware types on the hot-path. This has been changed by type matching at transform time that will be more efficient.

Additional tests have been added for the full context propagation cases and also to avoid reintroducing perf regressions because of reading the reactor context each time in the hot path.

Motivation

Additional Notes

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Jira ticket: [PROJ-IDENT]

[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	14.04 s	13.89 s	[+0.2%; +2.1%] (maybe worse)
startup:insecure-bank:tracing:Agent	12.87 s	12.93 s	[-1.6%; +0.7%] (no difference)
startup:petclinic:appsec:Agent	16.41 s	16.33 s	[-0.5%; +1.5%] (no difference)
startup:petclinic:iast:Agent	15.91 s	16.71 s	[-13.1%; +3.4%] (unstable)
startup:petclinic:profiling:Agent	16.60 s	16.52 s	[-1.1%; +2.1%] (no difference)
startup:petclinic:tracing:Agent	15.85 s	15.83 s	[-1.3%; +1.5%] (no difference)

Commit: 9126140b · CI Pipeline · Benchmarking Platform UI

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Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.