fix(gateway): don't force-enable matrix-sdk via the metrics feature

[resumeparseeval]

Summary

The gateway's metrics feature lists moltis-matrix/metrics without the weak ? qualifier, so enabling metrics force-enables the optional moltis-matrix dependency — pulling the entire matrix-sdk into the build even when the Matrix channel is otherwise disabled.

Every other optional channel in the same list already uses the weak form:

metrics = [
  "dep:moltis-metrics",
  "moltis-chat/metrics",
  "moltis-discord?/metrics",
  "moltis-matrix/metrics",     # <-- the lone non-weak ref
  "moltis-metrics/sqlite",
  "moltis-msteams?/metrics",
  "moltis-nostr?/metrics",
  "moltis-signal?/metrics",
]

moltis-matrix is the only inconsistency. The fix makes it weak (moltis-matrix?/metrics): metrics now enables matrix's metrics only when matrix is independently enabled, instead of dragging it into matrix-free builds.

Why this matters

A build that wants metrics but no Matrix (--no-default-features --features "...,metrics" without matrix) currently still compiles matrix-sdk. Besides the wasted build time, on the repo's pinned nightly matrix-sdk 0.16 currently fails to compile (error: queries overflow the depth limit), so this unnecessary edge makes otherwise-valid matrix-free builds fail. No effect on the default feature set, which enables matrix anyway.

Validation

Completed (via cargo tree)

• cargo tree -p moltis-gateway --no-default-features --features metrics -i matrix-sdk → "nothing to print" (was previously pulling matrix-sdk v0.16.0).
• cargo tree -p moltis-gateway -i matrix-sdk (default features) → still resolves matrix-sdk — no regression for default builds.
• cargo metadata parses cleanly.

Remaining (CI / maintainer)

• just release-preflight / full build matrix.

Manual QA

Build the gateway with metrics but without Matrix and confirm matrix-sdk is absent from the compile set:

cargo build -p moltis-gateway --no-default-features --features metrics

[greptile-apps]

Greptile Summary

This PR updates the gateway metrics feature wiring.

• Matrix metrics now use weak optional-dependency forwarding.
• Metrics-only builds no longer activate the Matrix dependency.
• Default builds still enable Matrix metrics when Matrix is enabled.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• When Matrix is enabled, the weak feature reference still enables Matrix metrics.
• When Matrix is disabled, skipping Matrix metrics matches the missing Matrix dependency.

Important Files Changed

Filename	Overview
crates/gateway/Cargo.toml	Changes the Matrix metrics feature reference to match the weak forwarding pattern used by the other optional channel crates.

_{Reviews (1): Last reviewed commit: "fix(gateway): don't force-enable matrix-..." | Re-trigger Greptile}