[RFC - do not merge] custom spans: user-declared spans from USDT probes and function symbols

[mmat11]

DEMO - Not intended to merge as-is.
This PR is a demo / discussion driver for a feature. I'm opening it to gather feedback on the shape of the config, the UX and next steps.

What this adds

A new top-level custom_spans config block that lets users declare
otel spans without touching application code or writing native
instrumentation. Four target shapes:

• function_span: "sym" — entry uprobe + return-site uprobes; span =
  entry-to-return.
• function_noret: "sym" — attach an entry uprobe at an ELF symbol.
• usdt_span: "provider:base" — attach at <base>_start and <base>_end;
  emit a span whose duration is the interval between them.
• usdt_noret: "provider:probe" — attach one uprobe at a USDT marker;
  each fire emits a zero-duration span.

For Go function_span targets, OBI also auto-extracts every scalar
and string argument (including primitive fields of *Request-style
struct pointers) by walking the runtime type metadata — no attrs:
block needed. Works on stripped binaries.

ebpf:
  custom_spans:
    ttl: 1m
    spans:
      - name: order.place
        on: { function_span: "main.(*Order).Place" }

What needs to be reviewed

Mostly devdocs/custom-spans.md.

Specifically I'd like feedback on:

• Config structure — is the current shape ok?
• Feature set — is what's here enough? Should we add sampling,
  rate limiting, service selectors, ...?
• Language coverage — start with Go and add more languages in
  follow-up PRs?

Additional features / ideas welcome.

Example

This test was done on https://opentelemetry.io/docs/demo/services/checkout/ - https://github.com/open-telemetry/opentelemetry-demo/tree/main/src/checkout

custom_spans:
    ttl: 30s
    spans:
      - name: checkout.place_order
        on:
          function_span: "main.(*checkout).PlaceOrder"
      - name: checkout.charge_card
        on:
          function_span: "main.(*checkout).chargeCard"
      - name: checkout.ship_order
        on:
          function_span: "main.(*checkout).shipOrder"
      - name: checkout.send_confirmation
        on:
          function_span: "main.(*checkout).sendOrderConfirmation"
      - name: checkout.prep_order_items
        on:
          function_span: "main.(*checkout).prepOrderItems"

place_order span (https://github.com/open-telemetry/opentelemetry-demo/blob/main/src/checkout/genproto/oteldemo/demo.pb.go#L1456):

[codecov]

Codecov Report

❌ Patch coverage is 69.09963% with 580 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.57%. Comparing base (3ba13ca) to head (706f480).
⚠️ Report is 169 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/ebpf/instrumenter.go	54.88%	92 Missing and 28 partials ⚠️
pkg/internal/ebpf/generictracer/generictracer.go	72.93%	45 Missing and 14 partials ⚠️
pkg/ebpf/custom_span.go	69.83%	44 Missing and 10 partials ⚠️
pkg/internal/gometa/recipe.go	66.19%	38 Missing and 10 partials ⚠️
pkg/internal/gometa/walker.go	66.94%	30 Missing and 9 partials ⚠️
.../internal/ebpf/generictracer/custom_span_reader.go	79.23%	33 Missing and 5 partials ⚠️
pkg/internal/gometa/method.go	54.87%	26 Missing and 11 partials ⚠️
pkg/internal/gometa/type.go	62.36%	28 Missing and 7 partials ⚠️
pkg/internal/gometa/field.go	57.33%	23 Missing and 9 partials ⚠️
pkg/ebpf/auto_attrs.go	77.34%	23 Missing and 6 partials ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2552      +/-   ##
==========================================
+ Coverage   69.23%   69.57%   +0.34%     
==========================================
  Files         350      362      +12     
  Lines       47637    49374    +1737     
==========================================
+ Hits        32980    34354    +1374     
- Misses      12577    12835     +258     
- Partials     2080     2185     +105     

Flag	Coverage Δ
integration-test	50.52% <42.84%> (-0.40%)	⬇️
integration-test-arm	29.35% <40.80%> (+2.43%)	⬆️
integration-test-vm-5.15-lts	29.72% <42.28%> (+0.38%)	⬆️
integration-test-vm-6.18-lts	30.23% <42.28%> (+2.30%)	⬆️
k8s-integration-test	34.51% <1.76%> (-1.13%)	⬇️
oats-test	33.95% <1.76%> (-1.77%)	⬇️
unittests	63.14% <49.46%> (-0.54%)	⬇️

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[grcevski]

Super excited for this @mmat11 !!! 🥇