update waffle flag custom metrics (#24270)

The previous version of this code used the Django Setting
ENABLE_WAFFLE_FLAG_METRIC to determine whether to add a single
metric with a dict of details about all flags. Due to
NewRelic's 256 character limit on the metric value, this was
getting truncated.

This new version instead uses the Django Setting
WAFFLE_FLAG_CUSTOM_METRICS, a list of waffle flag names to
instrument.

The name of each custom metric will match the name of the flag.
The value of the custom metric could be False, True, or Both.

The value Both would mean that the flag had both a True and False
value at different times during the transaction. This is most
likely due to having a check_before_waffle_callback, as is the
case with CourseWaffleFlag.

ARCHBOM-132