Added Unix epoch timestamp support for JSON parser

[drenizg]

closes: #4607

If parsing a Unix epoch timestamp in seconds, e.g. 1536092344.1, json_time_format config must be set to "unix" (case insensitive); corresponding JSON value can have a decimal part and can be a string or a number JSON representation.
If value is in number representation, it'll be treated as a double precision float, and could have some precision loss. If value is in string representation, there'll be no precision loss up to nanosecond precision. Decimal positions beyond that will be dropped.
If parsing a Unix epoch timestamp in milliseconds, e.g. 1536092344100, this config must be set to "unix_ms" (case insensitive); corresponding JSON value must be a (long) integer, and can be in number or string JSON representation.

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[danielnelson]

Thanks for the pr, can you rename the special value "unix" or make it case insensitive to keep style with the rest of the config? It should handle only seconds, not ms, but could support a decimal value like 1536095663.123. Since you won't be able to parse a time in ms you could add a "unix_ms" variation for your use case.

[drenizg]

Hi @danielnelson, thanks for the feedback! I just commited the requested changes.

[danielnelson]

Let's add support for string fields before we merge? More details in my comments.

[danielnelson]

One last thing, now that we have quite a bit of code for parsing the timestamps I think we should extract it into a new function.

[drenizg]

Agreed. Just commited the change. Now there's a new function called parseUnixTimestamp with all the parsing logic.

[Thidjs]

I'm breaking my head over this. When I use the "unix" time_format, i get the following error;
parsing time "1536303994" as "unix": cannot parse "1536303994" as "unix"
Why can't it parse this timestamp? And more important how to fix ;-)

[drenizg]

Hi @Thidjs, are you sure you're using a version with this change in it? If using a nightly build, it could be that it was packaged before the change was merged.

[Thidjs]

@drenizg I tested again today with the latest nightly build and it seems to be working now. Thanx!