docs(parsers): update readme for json_v2 parser

[mskonovalov]

Summary

related to #15837

switching parser from "json" to "json_v2" is not equivalent.
Not sure if this is expected behavior or not.
In my config I have only

data_format = "json"

If I switch it to

data_format = "json_v2"

then Telegraf just stops working and not printing any output.
It is very confusing, even though it may be expected.

Checklist

• No AI generated code was used in this PR

Related issues

resolves #

[Hipska]

!retry-failed

[srebhan]

@mskonovalov I'm not sure if we should put this note here... This general statement is true for any of the parsers as none of them has the same "semantics" compared to any other parser. So what should a user learn from this? To read and understand the documentation of the parser in charge?

If you do have some concrete hints for the user, e.g. "you need to explicitly define tags and fields" or similar, please feel free to add them but a "hey this is different than XYZ" won't help anyone IMO...

[Hipska]

What could be done instead is log a message on startup saying there is nothing configured for this parser. (or maybe even halt?)

[srebhan]

@Hipska that would be even better I guess. Not sure how easy it is, json_v2 code is a mess.

[Hipska]

@srebhan looks like it just needs some extra checks in func (p *Parser) Init() error

[mskonovalov]

@srebhan yeah, I agree it is not ideal for sure.

I guess the assumption came from the fact that json and json_v2 do exactly the same. So "v2 is like better option of v1" :)
Naming is just a bit misleading in that sense.

But i aggree that it might be better to fail fast if there is no configuration for v2.

[mskonovalov]

#15844

[srebhan]

Let's continue with PR #15844 and close this one...