[PoC] Support linking spans connected with Go chan

[MrAlias]

This PoC adds Go span linking for work handed between goroutines through a chan.

When OBI observes a matched channel send/receive, it captures the active OBI trace context on the sending and receiving goroutines in eBPF space and emits a dedicated channel-link event. User-space holds that link until the corresponding OBI spans are decoded, then exports the resolved spans with OTLP span links.

What Changed

• Added Go runtime uprobes for channel handoff detection:
  • runtime.chansend1
  • runtime.chanrecv1
  • runtime.chanrecv2
• Added eBPF-side handoff correlation for direct and buffered channels.
• Added a dedicated EVENT_GO_CHANNEL_LINK ringbuf event for cross-goroutine handoffs.
• Added user-space pending-link reconstruction keyed by trace_id + span_id.
• Attached reconstructed links to resolved OBI spans before OTLP export.
• Added a runnable example in examples/go-channel-links using docker compose, local OBI build, and Grafana LGTM.
• Updated docs to describe the actual design and the example workflow.

Behavior

Given two separate goroutines with active OBI spans:

• sender goroutine sends work through a Go chan
• receiver goroutine receives that work
• OBI exports the resulting spans as separate traces with reciprocal span links

In the example, this shows up as links between the OBI HTTP server spans for:

• GET /dispatch
• GET /receive

Known Limitations

• This PoC covers direct runtime channel wrappers only:
  • runtime.chansend1
  • runtime.chanrecv1
  • runtime.chanrecv2
• select-based channel paths are not covered yet; those go through runtime.selectgo
• The current example intentionally uses a plain blocking receive (<-workCh) for that reason
• This is focused on linking active OBI spans, not app-created manual Go SDK spans

[codecov]

Codecov Report

❌ Patch coverage is 40.79602% with 119 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.09%. Comparing base (6fc1474) to head (16c1f19).

Files with missing lines	Patch %	Lines
pkg/ebpf/common/go_channel_link_transform.go	55.29%	28 Missing and 10 partials ⚠️
examples/go-channel-links/app/main.go	0.00%	34 Missing ⚠️
pkg/internal/ebpf/gotracer/gotracer.go	36.58%	25 Missing and 1 partial ⚠️
pkg/ebpf/common/common.go	36.66%	19 Missing ⚠️
pkg/export/otel/tracesgen/tracesgen.go	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1727      +/-   ##
==========================================
+ Coverage   45.08%   45.09%   +0.01%     
==========================================
  Files         339      341       +2     
  Lines       37298    37484     +186     
==========================================
+ Hits        16814    16902      +88     
- Misses      19423    19509      +86     
- Partials     1061     1073      +12     

Flag	Coverage Δ
integration-test	20.86% <29.16%> (+0.04%)	⬆️
integration-test-arm	0.00% <0.00%> (ø)
integration-test-vm-x86_64-5.15.152	0.00% <0.00%> (ø)
integration-test-vm-x86_64-6.10.6	0.00% <0.00%> (ø)
k8s-integration-test	2.10% <0.00%> (-0.02%)	⬇️
oats-test	0.00% <0.00%> (ø)
unittests	48.55% <41.01%> (-0.01%)	⬇️

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