Add %kvp to default logging pattern to support SLF4J 2.x fluent key-value logging

[hanouard]

Description

Spring Boot 3 uses SLF4J 2.x, which introduced the fluent logging API (log.atInfo().addKeyValue(...).log(...)).
Currently, when using the default logging pattern, the key–value pairs are silently ignored because %kvp is not included in the log pattern.

This PR updates the default console and file logging pattern to append %kvp after the log message (%m).

Why

Developers using SLF4J’s fluent API expect key–value pairs to be visible without custom Logback configuration.

Aligns Spring Boot defaults with SLF4J 2.x features.

Provides more observability by making structured key–value logging visible out-of-the-box.

Changes

log.atInfo()
    .addKeyValue("userId", "42")
    .addKeyValue("action", "login")
    .log("User action performed");

Before:

2025-09-11 16:30:05.123  INFO --- [main] c.e.Demo : User action performed

After:

2025-09-11 16:30:05.123  INFO --- [main] c.e.Demo : User action performed userId="42" action="login"

[wilkinsona]

Thanks for the proposal. We discussed this and decided that this isn't something that we want to enable by default as we think the risk of logging secret values is too high. Logback has maskedKvp for this reason.

[hanouard]

Hello @wilkinsona, I do understand that's always risky to log passwords. But Isn't the responsability of the developer to make sure that no sensitive data is logged using the fluent api or classic one?

[wilkinsona]

We don't think it's solely the responsibility of the developer and that we can help by picking defaults that minimise the risks when a mistake is made. We feel it's too much to include them in console logging by default.