feat(core-nodes): emit df_engine.component.shutdown event from OTLP exporters

[cijothomas]

Adds a structured shutdown event to the OTLP gRPC and HTTP exporters, modeled after the OTel SDK otel.sdk.component.shutdown convention (open-telemetry/semantic-conventions#3723).

Changes

• In-tree semconv event definition (semconv/groups/event.component.shutdown.yaml) with full attribute declarations (otel.component.type, otel.component.shutdown.result, otel.component.shutdown.duration).
• OTLP gRPC exporter — replaces bare otel_info!("otlp.exporter.grpc.shutdown") with timed structured event.
• OTLP HTTP exporter — same pattern.
• Trigger script (99_df_engine.component.shutdown.sh) — POSTs to admin /api/v1/groups/shutdown?wait=true so the event fires in CI before the hard-kill. Prefixed 99_ to ensure it runs last in the trigger glob.

Notes

• result is always success today (the drain loop runs to completion). A follow-up can check the deadline to detect timed_out.
• duration is measured from message receipt, not from shutdown initiation (TODO in code — requires threading initiated_at through NodeControlMsg).
• df_engine prefix is provisional (product name unsettled; TODO in YAML).

Related

• #2325 — Handle OS signals (SIGTERM/SIGINT) for graceful pipeline shutdown. Once that lands, the shutdown event will fire naturally on SIGTERM without needing the admin API trigger.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.19%. Comparing base (2785c4f) to head (569c857).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3349      +/-   ##
==========================================
- Coverage   86.20%   86.19%   -0.01%     
==========================================
  Files         739      739              
  Lines      290493   290493              
==========================================
- Hits       250420   250393      -27     
- Misses      39549    39576      +27     
  Partials      524      524              

Components	Coverage Δ
otap-dataflow	87.25% <ø> (-0.02%)	⬇️
query_engine	89.58% <ø> (ø)
otel-arrow-go	52.45% <ø> (ø)
quiver	92.19% <ø> (ø)

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