add http header log when query error or slow query occurs

[baosenle]

Describe Your Changes

add http header log when query error or slow query occurs to vmselect. It can be configured to -log.httpHeaders=X-Webauth-User1,...,X-Webauth-UserN.

Checklist

The following checks are mandatory:

• My change adheres VictoriaMetrics contributing guidelines.

[baosenle]

issue: #6181

[baosenle]

Hi, @valyala @zekker6 @hagen1778
Can you take a look at this PR?
Thanks.

[hagen1778]

@baosenle thanks for PR!
I like the idea, many people use headers to identify the client or subset of clients when IP address is just not enough.
But I think the implementation of this feature should also account for the following settings that also print the log messages about the client:

• search.logQueryMemoryUsage
• client has cancelled the request after %.3f seconds message

The flag name "log.httpHeaders" should be aligned with search.log naming pattern.
The implementation is missing tests and documentation changes for https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/README.md