Avoid unnecessary string copy when no log level is specified

[hnw]

I'm using hashicorp/logutils with a library that requires log.Logger. I think it is very useful because we can use it as a library logger. Thank you very much!

However, I noticed a problem when using hashicorp/logutils with the library. If no log level is specified in the library log output, the log level will be considered a huge string and a huge string copy may occur. For example, this can be a problem when logging JSON like {"blocks": [...]}

So I introduced a maximum length of log level string to avoid huge string copy. I think this fix never cause any functional or performance issues with existing use cases.

In addition, this fix makes it easier to treat logs without a log level as log level "". This is a bit tricky, but it's useful if we use hashicorp/logutils to handle both our own logs and library logs.

[hashicorp-cla]

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