docs(experimental): reviewer-focused documentation for Metrics API/SDK (spec crosswalks, architecture, review guides)

[peerst]

• Why: Enable non-Erlang reviewers to validate implementation against the OpenTelemetry Metrics spec without diving into Erlang code.

• What: Adds complete, reviewer-friendly documentation across the experimental API and SDK:

  • API (apps/opentelemetry_api_experimental/)
    • SPEC_COMPLIANCE.md
    • ARCHITECTURE.md
    • REVIEW_GUIDE.md
    • GLOSSARY.md
    • TEST_COVERAGE.md
    • README updated with a “For non-Erlang reviewers” section
  • SDK (apps/opentelemetry_experimental/)
    • SPEC_COMPLIANCE.md
    • ARCHITECTURE.md (with sequence diagrams)
    • REVIEW_GUIDE.md
    • CONFIG_RECIPES.md
    • GLOSSARY.md
    • TEST_COVERAGE.md
    • README updated with a “For non-Erlang reviewers” section

• How to review

  • Start with apps/opentelemetry_api_experimental/SPEC_COMPLIANCE.md, then apps/opentelemetry_experimental/SPEC_COMPLIANCE.md.
  • Skim ARCHITECTURE.md in both apps for component responsibilities and flows.
  • Use apps/opentelemetry_experimental/REVIEW_GUIDE.md and CONFIG_RECIPES.md to validate behavior end-to-end (console reader recommended).

• Highlights

  • Clear spec-to-code crosswalk with code anchors for views, readers, aggregations, temporality, and observables.
  • Concise quickstart validation steps for each instrument type and common configurations.
  • Glossaries to map Erlang/OpenTelemetry terminology to language-agnostic concepts.

• Scope

  • Documentation only; no runtime behavior changes.

• Follow-ups (optional)

  • Add @doc summaries on key exported functions in otel_view, otel_metric_reader, and otel_meter_server to align inline docs with these files.

• Checklist

  • [ ] Links render correctly on GitHub
  • [ ] Mermaid diagrams render in ARCHITECTURE.md
  • [ ] Spelling/terminology pass
  • [ ] Team sign-off on doc structure/content

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