otelslog: Limit the size of kvBuffer returned to the pool

[MrAlias]

Limit the returned size so the pool doesn't hold on to very large buffers. Similar to https://cs.opensource.google/go/x/exp/+/814bf88c:slog/internal/buffer/buffer.go;l=27-34

Integration benchmark testing needs to be added along with the change to validate the performance improvement. Or this issue can be closed when that validation fails.

opentelemetry-go-contrib/bridges/otelslog/handler.go

Lines 357 to 359 in 59ebbee

	// TODO: limit returned size so the pool doesn't hold on to very large
	// buffers. Idea is based on
	// https://cs.opensource.google/go/x/exp/+/814bf88c:slog/internal/buffer/buffer.go;l=27-34