Neilj/fix 4229

[neilisfragile]

Metric synapse_admin_mau:current is stuck at 0 when config.mau_stats_only is True, where it should track the number of mau users

Fixes #4229

[codecov-io]

Codecov Report

Merging #4233 into develop will increase coverage by 0.01%.
The diff coverage is 90.32%.

@@             Coverage Diff             @@
##           develop    #4233      +/-   ##
===========================================
+ Coverage    73.55%   73.56%   +0.01%     
===========================================
  Files          299      299              
  Lines        29806    29821      +15     
  Branches      4872     4877       +5     
===========================================
+ Hits         21923    21939      +16     
+ Misses        6456     6451       -5     
- Partials      1427     1431       +4

Impacted Files	Coverage Δ
synapse/app/homeserver.py	56.95% <0%> (ø)	⬆️
synapse/storage/monthly_active_users.py	96.7% <100%> (+0.11%)	⬆️
synapse/api/auth.py	90.68% <100%> (+0.02%)	⬆️
synapse/handlers/register.py	70.86% <100%> (+0.12%)	⬆️
synapse/storage/prepare_database.py	74.67% <100%> (ø)	⬆️
synapse/api/constants.py	100% <100%> (ø)	⬆️
synapse/handlers/user_directory.py	71.42% <90%> (+0.39%)	⬆️
synapse/util/file_consumer.py	80.7% <0%> (-1.76%)	⬇️
synapse/state/v1.py	90.69% <0%> (-1.56%)	⬇️
synapse/handlers/search.py	81.25% <0%> (ø)	⬆️
... and 3 more

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[richvdh]

I think this is built on top of #4141, which has a bunch of outstanding comments, so I'm going to unflag this for review pending a successful merge of #4141 (or a separation of the PRs)

[richvdh]

@neilisfragile please could you also give the PR a more helpful summary?

[neilisfragile]

closed in favour of #4305