add grafana annotation limits

[rschitz]

Configures for how long alert annotations are stored. Default is 0, which keeps them forever.
This setting should be expressed as a duration. Examples: 6h (hours), 10d (days), 2w (weeks), 1M (month).
;max_annotation_age =

Configures max number of alert annotations that Grafana stores. Default value is 0, which keeps all alert annotations.
;max_annotations_to_keep =