A Service-Level Indicator (SLI) describes the "performance" of a
service. For some services, the SLI is well-defined. In such cases,
the SLI can be described easily by referencing the well-known SLI and
providing the needed parameters. Alternatively, a "custom" SLI can be
defined with a query to the underlying metric store. An SLI is defined
to be good_service / total_service over any queried time interval.
The value of performance always falls into the range 0 <=
performance <= 1. A custom SLI describes how to compute this ratio,
whether this is by dividing values from a pair of time series, cutting
a Distribution into good and bad counts, or counting time windows
in which the service complies with a criterion. For separation of
concerns, a single Service-Level Indicator measures performance for
only one aspect of service quality, such as fraction of successful
queries or fast-enough queries.
.. attribute:: type
Service level indicators can be grouped by whether the "unit"
of service being measured is based on counts of good requests
or on counts of good time windows
