feat: add HTTP_HOST to bind metrics/health server to a specific interface

[lystopad]

Description

Adds a new optional HTTP_HOST environment variable that lets operators bind the metrics/health HTTP server to a specific network interface (e.g. 127.0.0.1 for loopback-only, or a private interface) instead of having Node.js listen on all interfaces by default.

This is useful when the ejector runs on a host that should not expose its metrics/health endpoints publicly — operators can scope the server to a private interface and rely on a reverse proxy or local scraper for access.

Behavior

• HTTP_HOST is optional. When unset, behavior is unchanged: Node.js listens on all interfaces (:: on IPv6-capable hosts, otherwise 0.0.0.0).
• Accepts IPv4 (e.g. 127.0.0.1, 0.0.0.0) or IPv6 (e.g. ::1, ::) addresses.
• Only listen(HTTP_PORT, HTTP_HOST) is called when HTTP_HOST is set; otherwise the original listen(HTTP_PORT) call is preserved, so default Node.js binding semantics are kept identical.

Files changed

• src/services/config/service.ts — parse HTTP_HOST (optional string, no default).
• src/services/http-handler/service.ts — pass host to listen() only when defined.
• sample.env, sample.infra.env — commented-out example with IPv4/IPv6 guidance.
• README.md — new row in the env var table documenting the variable and its default.

Related Issue/Task

No related issue. Searched open issues for HTTP_HOST, bind, interface, 0.0.0.0 — no prior art.

How Has This Been Tested?

• Local tests (e.g., yarn test)
• Manual testing (described below)
• Not tested

Manual reasoning rather than runtime verification: the change is minimal and the conditional preserves the exact original .listen(HTTP_PORT) call when HTTP_HOST is unset, so existing configurations are unaffected. The new branch passes both port and host to Node's net.Server.listen(port, host), which is the documented signature for binding to a specific interface.

Happy to add an integration test for the bind address if maintainers prefer.

Checklist

• Documentation updated (README env var table + sample envs)
• New tests added (not added — no existing tests cover http-handler bind; can add on request)