feat: adding WithCustomMessage

[samber]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (9a4d16e) to head (ab68056).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

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##   main   #87   +/-   ##
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[Copilot]

Pull request overview

This PR introduces a new configuration hook to let callers override the log message produced by the Echo slog logging middleware.

Changes:

• Add WithCustomMessage callback to Config.
• Invoke WithCustomMessage (when provided) to override the final log message.
• Document the new Config field in the README config snippet.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
middleware.go	Adds WithCustomMessage to the middleware config and uses it to override the logged message.
README.md	Updates the documented Config struct to include WithCustomMessage.

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[Copilot]

Adding an exported field to Config is a source-breaking change for consumers who use unkeyed struct literals (slogecho.Config{...}) because the field order changes. This may conflict with the README promise of no breaking changes before v2; consider an alternative extension mechanism (e.g., functional options) or explicitly bump the major version / document that only keyed literals are supported.

[Copilot]

WithCustomMessage is passed errMsg, which is sometimes replaced with errors.New(httpErr.Message) for *echo.HTTPError (string message). That means the callback can’t inspect the original *echo.HTTPError (code/message/internal) or any wrapped/internal error; if the intent is to let users customize based on the real error, pass err (or httpErr) instead, or rename/adjust the callback contract to make it clear it receives a sanitized error.

Suggested change

	msg = config.WithCustomMessage(c, errMsg)
	msg = config.WithCustomMessage(c, err)