add a generic alert that is triggered when the number of logs being sent suddenly drops

related to: https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9522

• remove Gitaly specific low send rate alert with hardcoded threshold
• remove the corresponding runbook
• add a generic alert that is based on a derivative (until we can switch to comparing fluentd input and output: https://github.com/fluent/fluent-plugin-prometheus/issues/62)
• put all logging related alerts in one group (file)

Signed-off-by: Michal Wasilewski mwasilewski@gitlab.com