perf: check the log level before building log arguments

[chaikivskyi]

Reason for This PR

writeLog built the full argument list for every request before handing it to slog, so the work was performed even when the configured log level discarded the record. On a deployment running logs.level: warn or error - where no access log is ever emitted - each request still paid for r.URL.String(), start.Format(...), the stripCRLF calls, etc.

Description of Changes

writeLog now returns early when l.log.Enabled(context.Background(), slog.LevelInfo) is false, before any argument is built.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.