otel: Reactive Flush Instrumentation

[schloerke]

Implement reactive flush instrumentation to create reactive.update spans that wrap each flush cycle.

Goals

• Add reactive.update spans that wrap each flush cycle
• Store span on ReactiveEnvironment instance for child reactive spans to reference
• Span should include both sequential flush and flushed callbacks
• Respect REACTIVE_UPDATE collection level

Implementation

Core Changes

• Modify ReactiveEnvironment.flush() to wrap in span
• Store span on instance (_current_otel_span) for child spans to access
• Use save/restore pattern for nested flush support

Architecture Decision

• Refactored from module-level contextvar to instance variable
  • Better encapsulation and single source of truth
  • Easier access in Phase 4 for reactive calc/effect/output spans
  • Renamed to _current_otel_span for generality

Testing

• Collection level controls (REACTIVE_UPDATE, ALL, SESSION, NONE)
• Span creation during flush
• No-op when not collecting
• Instance variable storage and reset
• Parent-child relationship verification (session.start → reactive.update)

Acceptance Criteria

• Flush cycles create reactive.update spans when SHINY_OTEL_COLLECT >= reactive_update
• Span wraps all context flushes and flushed callbacks
• Multiple flush cycles create separate spans
• Span stored on ReactiveEnvironment instance
• All 55 OTel tests pass

Related

• Phase 2: otel: Session Lifecycle Instrumentation #2133
• Phase 4: otel: Reactive Execution Instrumentation #2135
• PR: feat(otel): Phase 3 - Reactive Flush Instrumentation #2146

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com