infra: drop OTel collector container — backend hosts OTLP directly (HOL-21)

[BrewingCoder]

Summary

The OTel collector was a SaaS-shaped multi-protocol relay (fluentforward, syslog, AWS Firehose CloudWatch, OTLP gRPC/HTTP/HTTPS, TCP log) that for self-hosted deployments only delivers OTLP HTTP value the backend already provides natively.

The .NET backend has a complete OTLP HTTP receiver at /otel/v1/{logs,traces,metrics}: JSON + protobuf, gzip + snappy decompression, CORS-enabled, project-id resolution from resource attributes. Self-instrumentation already targets it directly. SDKs just needed their default OTLP endpoint pointed at it.

Removed

• compose.yml + compose.hobby-dotnet.yml: collector service
• infra/docker/{collector.Dockerfile, collector.yml, configure-collector.sh}
• start-infra.sh: collector from SERVICES list
• .env.example: OTEL_COLLECTOR_* image vars
• env.sh: collector hostname rewrites

Updated

SDK + frontend defaults from :4318 (collector) to :8082/otel (backend):

• sdk/highlight-run/src/index.tsx: otlpEndpoint default
• compose.hobby-dotnet.yml + compose.dev-frontend.yml: REACT_APP_OTLP_ENDPOINT
• env.sh: OTLP_DOGFOOD_ENDPOINT, OTLP_ENDPOINT

Tradeoffs accepted

• Lose support for non-OTLP ingest sources (fluentforward, syslog, AWS Firehose). For self-hosted deployments these were never wired through to anything we use.
• Collector also offered protocol upgrades (gRPC ↔ HTTP). Backend only accepts OTLP HTTP. Anyone wanting gRPC ingest can run the collector themselves as an external relay.

Verified

• docker compose ps now shows 7 HoldFast containers (was 9 before HOL-19 + HOL-21).
• Smoke test (./infra/docker/smoke-test-ingest.sh) passes end-to-end.
• Backend's self-instrumentation continues to flow into ClickHouse via the in-process /otel handler (no behavior change there).

Stacks on #99 (HOL-19). Subtask of HOL-17. Closes HOL-21.

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