feat(otel): OTel Collector receiver plugin for Flagon JSON

[brucearctor]

Summary

Build a custom OTel Collector receiver (flagonreceiver) that accepts Flagon's existing JSON format and converts it to OTLP LogRecords server-side.

Motivation

For users who can't change the client (e.g., already deployed v2.x UserALE) but want OTel on the backend.

Design

• Written in Go (Collector plugin requirement)
• Accepts POST JSON on configurable HTTP path
• Maps Flagon JSON fields → OTLP LogRecords
• Registers as a standard Collector component
• Could be contributed to opentelemetry-collector-contrib

Structure

collector/flagonreceiver/
  factory.go
  receiver.go
  config.go
  README.md

Priority

Lower priority than client-side OTLP exporter (#140). Build when there's user demand.

Depends On

• feat(export): OTLP/JSON exporter for OpenTelemetry integration #140 (OTLP exporter — proves the attribute mapping works)