Feature/runtime logs

[i-hate-nicknames]

Did you run make format && make check?

Fixes #725

Changes:

• Add GET /api/visors/{pk}/runtime-logs endpoint

How to test this PR:
Run visor with hypervisor. Check output of the request.

Response is an array of json-encoded log entries. Entries are created by logger clients and can have custom fields with values, e.g. PK field that provide additional info.
Each log entry should have the following fields:
time: formatted timestamp of the log entry
msg: message of the entry
level: level of the entry
log_line: log entry line in the global log. Since the capacity of log store is limited, some old entries could be dropped.
All other keys may be absent in some entries.

[Senyoret1]

I found some things:

• I tested it and got 50+ elements in the response, but all of them had level set to panic. Some entries just had info about API responses, so I think there is a problem with that.

• The response appears to be JSON, but the format is not correct, because the elements are not really inside an array. Opening it with Firefox causes this error to appear:
  

Appart from that, the response is somewhat complex, as there are a lot of possible properties and not all entries have the same ones. If the manager is just going to open the whole response in a new tab, there is no problem, but if the manager needs to show the data in a more processed way then we will need to plan what will be shown, to check if the data currently being returned will allow to do that.

[i-hate-nicknames]

Thanks. Another issue that I just found is that not logs get to the hook for some reason. Apparently, new subloggers created by e.g. dmsg do not inherit hooks.

@Senyoret1 log entries are inherently heterogeneous, because callers add their own data that makes sense to them to the entries. JSON representation just gets all the data that is there.
I thought about returning just textual representation of an entry, but this way there will be no coloring and it might get long.
Would it be possible to use the generic parts of a log entry for presentation (level, date, log line, message), and show every other field as "additional fields" somewhere, like a dropdown?

[jdknives]

@i-hate-nicknames this endpoint and the returned logs do not need to be colored to be honest. We will mostly use this for diagnostics so we may even just return a file to the user with plain text. Colored and nicely formatted text in the UI would be cool but not really necessary so I would keep it simple.

[i-hate-nicknames]

@Senyoret1 ended up using json formatter, because standard text formatter only pretty-prints entries when using colors. I will update PR description with fields that are included in every entry

[jdknives]

Good job.