Warn users when lambda producer customizers are used

[onobc]

It may be helpful to add a warn log statement when users use a lambda for a producer customizer in hopes this can be caught in the logs. It is hard to notice it and expecting users to dig into the docs to understand or look at cache miss metrics to notice this is happening is not the best experience.

Thanks

Originally posted by @onobc in #806 (comment)

[onobc]

Closed via 473642e