[kotlin-client][jvm-spring-webclient] Fix metrics URI templating for WebClient

[paul-kraftlauget]

Fixes #16923

Spring configures the uriTemplate attribute and not generated openapi code. It also leverages Spring's uriBuilderFactory to correctly set the template.

Also, the commit removes a compile warning.

Kotlin technical committee:
@dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03) @stefankoppier (2022/06) @e5l (2024/10)

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