emit SpanNode and TaskBlock events for critical-path analysis

[kaahos]

What Does This Do

Introduces two new JFR event types emitted by the Datadog profiler to capture the causal structure of traced requests:

• SpanNode events, emitted on every span completion
• TaskBlock events, emitted on every LockSupport.park() blocking interval that occurs under an active span
• A LockSupportProfilingInstrumentation module that instruments LockSupport.park* / unpark to capture these edges.

Motivation

To improve latency attribution, critical-path analysis seems the way to go. It requires knowing which thread is the bottleneck for a given request at each point in time.

SpanNode and TaskBlock events together allow the backend to reconstruct the full execution DAG of a trace: SpanNode provides the span tree structure with precise timing, and TaskBlock provides the inter-thread wakeup edges (park -> unpark) needed to identify which thread is on the critical path.

LockSupport.park/unpark is the foundation of most JVM blocking primitives (ReentrantLock, CountDownLatch, CompletableFuture, virtual threads), so instrumenting it captures the majority of inter-thread handoffs in practice.

Additional Notes

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Jira ticket: PROF-12146

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