SMTP: Add metrics

[blind-oracle]

This adds Prometheus metrics to SMTP server & IC Delivery Agent.
Also adds message_id field to the Candid protocol.

[Copilot]

Pull request overview

Adds Prometheus instrumentation to the SMTP server (per-session counters, gauges, histograms for bytes, commands, replies, messages, errors and session duration) and to the IC delivery agent (canister-id lookup and IC SMTP request metrics). Also adds a message_id field to the Candid SmtpRequest, lowers the default max message size to fit IC limits, and adds two small reusable helpers (IpFamily, BoolYesNo).

Changes:

• New Metrics types in smtp::mod and smtp::ic, plumbed through Server, SessionManager, Session, and IcSmtpDeliveryAgent.
• Recording of bytes/commands/replies/protocol errors/session lifecycle in inbound SMTP, and canister-id lookup + per-request latency in the IC delivery agent.
• Candid protocol gains message_id: Option<String> and delivery now uses join_all so all canisters are observed even when some fail.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
ic-bn-lib/src/lib.rs	Adds IpFamily and BoolYesNo helper traits used by new metric label code.
ic-bn-lib/src/smtp/mod.rs	Defines inbound SMTP Metrics struct and derives IntoStaticStr on error enums for labels.
ic-bn-lib/src/smtp/cli.rs	Lowers default max message size to 1950KB to fit IC message limits.
ic-bn-lib/src/smtp/server.rs	Threads Metrics through Server::new/new_with_listener into sessions.
ic-bn-lib/src/smtp/inbound/mod.rs	Adds labels/metrics to Session, moves shutdown here, adds protocol-error/message metric updates and a request_str helper.
ic-bn-lib/src/smtp/inbound/session.rs	Records bytes_tx/bytes_rx, replies, and per-command counters in the state machine.
ic-bn-lib/src/smtp/inbound/manager.rs	Records open/processed sessions and durations, including a partial path on TLS handshake failure.
ic-bn-lib/src/smtp/inbound/tests.rs	Updates test call sites for new Metrics parameter.
ic-bn-lib/src/smtp/ic/mod.rs	Defines IC delivery Metrics struct (lookups + request latencies).
ic-bn-lib/src/smtp/ic/delivery_agent.rs	Threads Metrics through, switches to join_all, records canister-id lookup and SMTP request metrics, adds message_id to requests.
ic-bn-lib/src/smtp/ic/candid.rs	Adds message_id: Option<String> to SmtpRequest.

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[Copilot]

Pull request overview

Copilot reviewed 11 out of 11 changed files in this pull request and generated no new comments.