V12- fix(forwarder): set gRPC metadata type to collector instead of agent

[yllada]

Closes #2290

Summary

The forwarder was sending type: "agent" in the outgoing gRPC metadata context.
The log-auth-proxy rejects streams that don't identify as "collector", causing
an EOF loop that prevented any logs from being forwarded.

Changes

File	Change
collectors/forwarder/serv/service.go	Changed metadata type value from "agent" to "collector"

[github-actions]

🛑 AI review — Engineer review required

This PR touches critical paths or introduces changes the model cannot judge with sufficient confidence. @Kbayero @osmontero please review.

🛑 architecture (gemini-3-flash-lite) — blocking — must fix before merge

Summary: Modification of gRPC metadata 'type' field in forwarder service potentially breaks agent-manager authentication or routing logic.

• high collectors/forwarder/serv/service.go:91 — Changing the 'type' metadata from 'agent' to 'collector' alters the identification contract with the agent-manager. This may break authentication, authorization, or routing logic for existing deployments and requires a coordinated rollout plan.

✅ bugs (gemini-3-flash-lite) — clean

Summary: Minor change to outgoing context metadata; no functional bugs detected.

No findings.

✅ security (gemini-3-flash-lite) — clean

Summary: Change of metadata type from 'agent' to 'collector' in gRPC context; no security impact.

No findings.

[utmstackprapprover]

Changes requested — AI review found blocking issues (high/critical, or engineer review required). See above.