feat(o11y): self-tracing via runtime spans, adapter emit spans, and OTLP trace export (PIPE-1066)

[Dylan-M]

Proposed Change

Adds blitz's session-level self-tracing. The runtime emits a root blitz.session span
from Start to Stop, with a child blitz.generator.run span per module. Consumer adapters
emit gated blitz.emit.* spans. Trace export is opt-in through the new telemetry.traces
config. With an OTLP endpoint set, spans export there. Without one, the nil TracerProvider
means spans are created and dropped by the global no-op provider. PerBatchSpans gates
the higher-volume per-emit spans and is off by default.

Each setting is exposed on all three config surfaces, like every other blitz option: the
telemetry.traces config block, BLITZ_TELEMETRY_TRACES_* env vars, and
--telemetry-traces-* flags.

How to validate

• Point blitz at any OTLP gRPC collector (its debug exporter shows the spans). Run:
  go run ./cmd/blitz --generator-type apache-common --generator-apache-common-rate 1s --output-type stdout --telemetry-traces-otlpendpoint localhost:4317 --telemetry-traces-insecure
• Confirm a blitz.session span with a child blitz.generator.run span arrives at the
  collector.
• Drop the two --telemetry-traces-* flags and confirm blitz runs with no spans exported.
• go test ./internal/runtime/... ./internal/telemetry/traces/... ./output/... covers the
  span lifecycle, the OTLP exporter seams, and the emit-span gating.
• go test ./internal/config/... covers the flag, env, and default for each new setting.

Checklist

• Changes are tested
• CI has passed

[Dylan-M]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(o11y): record output request + HEC ACK-poll latency in ms (PIPE-1404) #292
• feat(o11y): lifecycle startup-latency metrics via new runtime weaver registry (PIPE-1066) #291
• feat(o11y): bring stdout/nop outputs fully into the self-telemetry spine (PIPE-1066) #290
• feat(o11y): per-batch output send spans + HEC ACK-poll span (PIPE-1066) #289
• feat(o11y): wire fix generator into the self-telemetry spine (PIPE-1066) #288
• feat(o11y): self-logs via shared entry-point zap->OTel bridge + OTLP log export (PIPE-1066) #287
• feat(o11y): self-tracing via runtime spans, adapter emit spans, and OTLP trace export (PIPE-1066) #286 👈 (View in Graphite)
• feat(o11y): per-instance metrics via injectable MeterProvider, drop package globals (PIPE-1066) #285
• feat(embed): add TelemetrySettings provider bundle + NopTelemetry (PIPE-1066) #284
• main

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