Investigate options and possible overhead of global string truncation

To ensure some user errors don't lead to overblown payload sizes, we want to truncate all strings in the payload to a max length, similar to what we already to with strings that are stored as keywords in Elasticsearch. This could potentially be quite overhead-heavy, so we need benchmarks of different approaches

* explicitly wrap all strings in a `maybe_truncate` function that either returns the string itself or, if necessary, a truncated version
* loop over the payload and truncate all strings before serializing
* modify `JSONSerializer` to truncate strings

从 JSONSerializer 和 issue 中描述的 payload 序列化路径开始。对列出的三种截断方法进行基准测试，包括它们的开销，并记录结果和推荐的方法；issue 未指定最大长度或测试目标。

由索引模型根据 Issue 内容生成。