apollo_committer: restore lazy log formatting for per-block commit stats

[gkaempfer]

Summary

Follow-up performance fix for #14888, which added a commit_duration_warn_threshold_millis-based WARN log to log_block_measurements (crates/apollo_committer/src/committer.rs). That change pre-built the block-stats message via a bare let stats = format!(...) before passing it into warn!(...)/debug!(...).

log_block_measurements runs once per committed block, on the committer's block-finalization path (a slow committer stalls consensus, per the existing comment). tracing's debug!/warn! macros are designed to be zero-cost when a level is disabled: the callsite's "interest" is checked before any of the macro's format arguments are evaluated, so formatting work is skipped entirely when that level isn't active. Building the message with a standalone format!() outside of the macro call defeats that — it unconditionally does the string formatting (5+ nested format! calls for optional fields) on every single block, even when the subscriber is at INFO level and neither branch will actually emit a line.

Fix

• Moved the message formatting into a format_block_stats closure invoked from inside the warn!/debug! macro args, restoring the callsite-interest laziness (only one of the two branches runs per call, and its formatting only happens if that level is enabled).
• This also happens to make the pre-existing witness_log formatting (under the os_input feature) lazy for the same reason.
• Net removal of duplicate code vs. formatting the message twice (once per branch).
• Strengthened no_warn_when_block_commit_within_duration_threshold to also assert the debug branch actually emits a log line (previously it only asserted the absence of the warn suffix).

Test plan

• cargo build -p apollo_committer
• SEED=0 cargo test -p apollo_committer — 12/12 passed, including both duration-threshold tests
• cargo clippy -p apollo_committer --all-targets — clean
• scripts/rust_fmt.sh — no diff
• Reviewed by an independent Opus pass, which verified the duplicated-then-deduped log message is byte-identical to the original and found no blocking issues

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Generated by Claude Code

[cursor]

PR Summary

Low Risk
Logging-only performance fix with identical message content; no commit or consensus logic changes.

Overview
Restores lazy formatting for per-block commit stats in log_block_measurements, which runs on every committed block.

The stats string is built inside a format_block_stats closure passed into warn! / debug! instead of a standalone format! before the macro, so tracing can skip work when that level is disabled (e.g. INFO with no debug/warn emission). The same pattern makes os_input witness stats lazy again. Log text is unchanged; the slow-commit branch inlines the warn threshold millis in the macro args.

The within-threshold test now also asserts the debug stats line (Block 0 stats) is present, not only that the warn suffix is absent.

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