[metrics-team] Monthly churn values per relay flag

[Philipp Winter]

I created relay churn diagrams per relay flag:
<https://nymity.ch/sybilhunting/churn-values/>

For those of you who haven't seen this kind of visualisation before:
The churn rate is a metric to capture the relay turnaround between two
subsequent consensuses.  It takes a value in the interval [0,1].  0
means that no relay joined or left.  1 means that all relays left and a
completely different set of relays joined the network.

In this visualisation, I calculated the churn rate between all
consensuses since 2007.  I plotted the churn values for relays with the
Guard, Exit, HSDir, Stable, Valid, and V2Dir flag.  Here are some
observations:

- The churn rate of the network has decreased slightly over the
  years.  Perhaps we now have a more stable network core.

- The time series has plenty of outliers, many of which are probably
  consensus hiccups.  It strikes me as difficult to determine the cause
  of outliers without digging deep into network status votes.

Cheers,
Philipp