Add logging policy (#4104)

* Add a doc defining logging policy for contributors

* Clarify wording and add msg send/rcv as INFO

docs/contributors/logging-policy.md

@@ -0,0 +1,68 @@
+# Logging policy
+
+This document defines what each log level means in Logos Messaging and how to
+choose a level for a new log statement. It exists so that levels carry a
+consistent, enforceable meaning for our two main audiences:
+
+- **Operators** run at `INFO`: node state and health, no noise.
+- **Developers** run at `DEBUG`: protocol interactions, no spam.
+
+## Guiding principle
+
+> **A healthy node logs nothing at WARN or above.**
+
+Every `WARN`/`ERROR`/`FATAL` line must indicate something to fix, investigate, or
+report. In review, ask: would a healthy node ever hit this line? If yes, it is
+not `WARN` or above.
+
+## Levels
+
+Increasing severity: `TRACE`, `DEBUG`, `INFO`, `NOTICE`, `WARN`, `ERROR`,
+`FATAL`.
+
+- **TRACE** — high-volume internals: per-message relay/gossipsub handling,
+  payloads, validation internals, loop ticks. Filtered to a topic when used.
+- **DEBUG** — the developer narrative: one line per protocol interaction
+  (request served, peer selected, dial result, retry attempt). Scales with
+  protocol activity, not raw throughput.
+- **INFO** — the operator narrative: lifecycle steps, one mount summary,
+  connection-state changes, one periodic health line, and messages sent or
+  received through the Messaging / Reliable Channels API. Not for
+  per-peer-event churn.
+- **NOTICE** — rare must-see lifecycle facts, visible above `INFO`: node
+  started (version, addresses, ENR), shutdown initiated. A handful per process.
+- **WARN** — degraded but recoverable, or config needing attention: exhausted
+  retries for an optional capability, suspicious-but-valid config, deprecated
+  options.
+- **ERROR** — local malfunction: a broken internal assumption or a
+  node-initiated operation that failed and impairs this node. Never caused
+  solely by remote-peer input.
+- **FATAL** — node cannot continue; process exit follows.
+
+## Decision rules
+
+1. **Remote/network anomalies** (invalid messages, malformed ENRs, unreachable
+   peers) — `DEBUG` (`TRACE` on the per-message path) plus a metric counter.
+2. **Retry loops** — each attempt `DEBUG` with attempt number; on exhaustion
+   `WARN` if an optional capability is degraded, `ERROR` if core function is
+   lost.
+3. **Persistent conditions** — log the transition, not every tick. Repeats go
+   to `DEBUG`.
+4. **API and idempotent guards** ("not mounted", "already started") — `DEBUG`.
+5. **Deprecated options** — config to be removed in future versions; `WARN`
+   once, at config-parse time.
+6. **Periodic status** — one consolidated `INFO` line per tick; detail to
+   metrics or `DEBUG`.
+7. **Hot paths** — request/response protocols `DEBUG` per request; relay/gossip
+   `TRACE` per message.
+
+## Style
+
+- Start the message with a capital letter.
+- Keep the message a constant with variable data in `key = value` fields,
+  including the cause on failures (`error = $err`).
+- Log an error at the boundary where it is handled, not before returning it.
+- One `logScope` topic per module.
+- No side effects or expensive computation in log arguments; lower levels may
+  be compiled out.
+- Give any demoted `WARN`/`ERROR` a metric counter if it was the only signal.