feat(observability): OTLP exporter + collector docs

[ALRubinger]

Summary

Adds the production trace destination Aileron was missing. With AILERON_OTEL_EXPORTER=otlp, spans ship to an OpenTelemetry Collector via OTLP/HTTP — the path that fans out to Grafana / Datadog / Honeycomb / Jaeger / Tempo / etc. without bespoke integration.

Closes the "configurable endpoint" goal of #390 (the issue itself was closed earlier; this completes the production deployment story it described).

Why HTTP, why no Aileron prefix

• OTLP/HTTP over gRPC — keeps the dep tree light and plays nicely with proxies. The SDK ships both; HTTP is the lower-friction default for self-hosters.
• Honors OTEL_EXPORTER_OTLP_* directly — no Aileron-prefixed alternatives. Anyone running an OTel collector already has these env vars in their environment; forking the names would force them to maintain two parallel sets.

Usage

Local dev:

docker run --rm -p 4318:4318 otel/opentelemetry-collector-contrib:latest

AILERON_OTEL_ENABLED=true \
AILERON_OTEL_EXPORTER=otlp \
OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318 \
OTEL_EXPORTER_OTLP_INSECURE=true \
aileron launch claude

Managed backends use OTEL_EXPORTER_OTLP_HEADERS for auth — Honeycomb, Grafana Cloud, Datadog all show concrete examples in the docs.

Docs restructure

This PR also restructures the Observability guide:

1. New "What is OpenTelemetry?" top-level section above the audit log, so readers without prior OTel familiarity get oriented before either surface is described. Defines OTel, traces/spans, OTLP, OTel endpoints, and the collector-fan-out value prop.
2. New "Hooking up to a collector" subsection with concrete docker run example for local dev plus Honeycomb and Grafana Cloud production examples.
3. Configuration table updated with the OTLP row and the standard OTEL_EXPORTER_OTLP_* env vars (headers, insecure, etc.) so users have everything in one reference.

Test plan

• TestOTLPExporter_PostsToCollectorEndpoint — full round-trip against an httptest server impersonating an OTLP/HTTP collector; asserts POST to /v1/traces with application/x-protobuf body
• TestNewExporter_DispatchesOTLPByConfig — config dispatch wiring
• TestLoadObservabilityConfig_AcceptsOTLPExporter — config parsing
• Coverage: internal/observability 90.7%, internal/config 96.1%
• go test ./... — full internal-module sweep green
• All four cmd/* modules build cleanly
• task build:docs — 55 pages, no errors

🤖 Generated with Claude Code

[railway-app]

🚅 Deployed to the aileron-pr-475 environment in aileron

Service	Status	Web	Updated (UTC)
docs	✅ Success (View Logs)	Web	May 6, 2026 at 7:19 am

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.10%. Comparing base (7987efa) to head (162d7f2).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #475      +/-   ##
==========================================
+ Coverage   82.24%   83.10%   +0.85%     
==========================================
  Files         234      234              
  Lines       24127    24129       +2     
==========================================
+ Hits        19843    20052     +209     
+ Misses       3148     2924     -224     
- Partials     1136     1153      +17     

Flag	Coverage Δ
integration	15.62% <20.00%> (+7.17%)	⬆️
unit	78.90% <100.00%> (+<0.01%)	⬆️

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