* always export max_age/min_age
* downgrade errors of missing data
on start with empty recorder database these errors are logged multiple times:
ERROR (MainThread) [homeassistant.components.sensor.statistics] mean requires at least one data point
ERROR (MainThread) [homeassistant.components.sensor.statistics] variance requires at least two data points
downgrade them to debug as they are not meaningful to end users
* add change_rate attribute
this calculates the average change rate of all data points
* simplify count, reorder attribute calculation
* reorder initialization
* reorder attribute names
* don't use min/max for min_age/max_age
* add test case
* style
* style
* sort constants
* init variables with None
* add precision config setting
* round to precision
* test round
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