Monitor users by time since last request

[nhoening]

This feature is useful for those user accounts which are expected to send/get data in automated fashion.

If they don't connect in the expected frequency, something's wrong - probably at their side. They will get alerts, just as the usual FlexMeasures operators.

± flexmeasures monitor last-seen --help                                                                
Usage: flexmeasures monitor last-seen [OPTIONS]

  Check if given users last contact (via a request) happened less than the
  allowed time ago.

  Helpful for user accounts that are expected to contact FlexMeasures
  regularly (in an automated fashion). If the last contact was too long ago,
  we send alerts via Sentry, as well as emails to monitoring mail recipients.
  The user can be informed, as well.

  The set of users can be narrowed down by roles.

Options:
  --maximum-minutes-since-last-seen INTEGER
                                  Maximal number of minutes since last
                                  request.  [required]
  --alert-users BOOLEAN           If True, also send an email to the user.
                                  Defaults to False, as these users are often
                                  bots.
  --account-role TEXT             The name of an account role to filter for.
  --user-role TEXT                The name of a user role to filter for.
  --custom-user-message TEXT      Add this message to the monitoring alert
                                  email to users (if one is sent).
  --help                          Show this message and exit.

[coveralls]

Pull Request Test Coverage Report for Build 3633712562

• 37 of 82 (45.12%) changed or added relevant lines in 6 files are covered.
• 131 unchanged lines in 8 files lost coverage.
• Overall coverage decreased (-0.03%) to 64.946%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
flexmeasures/cli/monitor.py	21	66	31.82%

Files with Coverage Reduction	New Missed Lines	%
flexmeasures/data/schemas/sources.py	2	81.25%
flexmeasures/data/services/scheduling.py	7	83.78%
flexmeasures/api/common/schemas/sensor_data.py	13	80.81%
flexmeasures/data/models/planning/utils.py	13	82.58%
flexmeasures/data/models/charts/defaults.py	19	52.04%
flexmeasures/data/models/charts/belief_charts.py	21	12.77%
flexmeasures/api/v3_0/sensors.py	28	70.47%
flexmeasures/cli/data_add.py	28	31.53%

Totals
Change from base Build 3546393110:	-0.03%
Covered Lines:	6643
Relevant Lines:	9592

---

💛 - Coveralls

[Flix6x]

I spent most of my review time on understanding why a potential timezone issue I noticed wasn't causing problems. Feel free to make a separate ticket for that, because I know that timezone issues can keep you occupied for a long time, and your code doesn't actually have a problem at this point.

My only actual concern is missing opt-in / opt-out functionality. Not asking to implement that, but some documentation warning the admin about sending emails to users without them being able to opt out seems prudent.

Otherwise, only a few (other) touches to the documentation.

[Flix6x]

Approved with grammar fixes.

[Flix6x]

Sidenote: Any reason why you didn't update the logic in server_now()? From your comment I figured you were enthousiastic about updating it in this PR.

[nhoening]

Actually, I overlooked the logic change you proposed. Thanks!