JSON parsing extension (DOC for rsyslog PR/#2773)

[nhosoi]

This is an update for rsyslog/rsyslog#2773 containing the following changes:

• Adding parameters variable, alt_variable and compact to mmjsonparse.
• Adding parse_json extension "json_parse_ex" which supports compact.

[nhosoi]

The build failed with this error... Could you please enlighten me what was wrong in this PR? Thanks!!
fatal: Couldn't find remote ref refs/pull/691/merge

[richm]

instead

[richm]

@nhosoi try rebasing on top of the latest master

[nhosoi]

Thank you, @richm. I thought I fixed it when I copied the section from mmnormalize, but I added another typo. :p Fixed both.

mmjsonparse.rst:   Specifies if a variable insted of property 'msg' should be used for
mmnormalize.rst:   Specifies if a variable insted of property 'msg' should be used for
==>
mmjsonparse.rst:   Specifies if a variable instead of property 'msg' should be used for
mmnormalize.rst:   Specifies if a variable instead of property 'msg' should be used for

Regarding "rebasing on top of the latest master", I've done that just before creating this pr. So, it's up to date...

[nhosoi]

/retest

[rgerhards]

The build failed with this error... Could you please enlighten me what was wrong in this PR? Thanks!!
fatal: Couldn't find remote ref refs/pull/691/merge

That's apparently a race in the github notification API. We see this occasionally. It looks like the commit is not actually available on all github front-end servers at the time the notification is sent out. If buildbot then starts up very quickly, we can hit that problem.

We are investigating how to work-around it. Most probably, we will just add an initial "sleep step" to the buildbot configs.

[nhosoi]

Closing this Doc PR along with the primary PR.
rsyslog/rsyslog#2773

Thank you for the discussions, @davidelang, @rgerhards, and @richm.
I've submitted a pr for skipempty to liblognorm. I'd greatly appreciate it if you could take a look at it.
rsyslog/liblognorm#305

Since some of the necessary features are already in mmnormalize and the rest could be added by the liblognorm pr305, let me close this pr as well as the doc pr (DOC for rsyslog PR/#2773) #691.