feat(o11y): per-batch output send spans + HEC ACK-poll span (PIPE-1066)

[Dylan-M]

Proposed Change

Adds send-latency tracing to the outputs, gated by PerBatchSpans. tcp, udp, and file
carry the emit-span context through their internal channel and parent a per-message span
on the actual send: blitz.output.tcp.send, blitz.output.udp.send, and
blitz.output.file.write. otlp and hec batch many records, so they emit a standalone
send span carrying blitz.batch.size rather than parenting to one record. hec adds a
blitz.output.hec.ack_poll span around the indexing-confirmation poll. syslog delegates
to its tcp/udp transport, so it is not double-spanned.

How to validate

• Start a TCP listener (nc -lk 5140), point blitz at a collector, and enable per-batch
  spans:
  go run ./cmd/blitz --generator-type apache-common --generator-apache-common-rate 1s --output-type tcp --output-tcp-host localhost --output-tcp-port 5140 --telemetry-traces-otlpendpoint localhost:4317 --telemetry-traces-insecure --telemetry-traces-perbatchspans
• Confirm per-message blitz.output.tcp.send spans arrive as children of the emit span.
• Swap the output flags for hec to see the batch span plus the ACK-poll span:
  --output-type hec --output-hec-host localhost --output-hec-port 8088 --output-hec-token test-token --output-hec-enableack.
  Confirm a blitz.output.hec.send span carrying blitz.batch.size, plus a
  blitz.output.hec.ack_poll span.
• Drop --telemetry-traces-perbatchspans (the default) and confirm no send spans are
  emitted.
• go test ./output/... covers each output's send span, the off path, and the otlp/hec
  batch spans via mock service clients.

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 👈 (View in Graphite)
• 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
• 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

This stack of pull requests is managed by Graphite. Learn more about stacking.