Add saturation monitoring back for redis-cache
In #1746 (closed), we removed the maxmemory saturation for redis-cache, as redis-cache was always at that value.
Now, with &764 (closed), it is not! We also have a saturation point that sounds exactly like what we could use here, added for redis-sessions: https://gitlab.com/gitlab-com/runbooks/-/blob/master/libsonnet/saturation-monitoring/redis_memory.libsonnet#L25-43
// How much of maxmemory (if configured) Redis is using; relevant
// for special cases like sessions which have both maxmemory and eviction, but
// don't want to actually reach that and start evicting under normal circumstances
We should consider adding that back for redis-cache. The downside is that this is much more spiky than redis-sessions: because the TTL is low, weekend memory usage is lower, for instance. Prophet should handle this https://facebook.github.io/prophet/:
Prophet is a procedure for forecasting time series data based on an additive model where non-linear trends are fit with yearly, weekly, and daily seasonality, plus holiday effects
I think that we should default to adding this saturation monitoring back to Tamland, then remove it if it's still too noisy.