Refactor monitoringThresholds and nodeLevelMonitoring to monitoring object in metrics catalog

The following discussion from !6394 (merged) should be addressed:

• @sxuereb started a discussion: (+1 comment)

  question (non-blocking): Will we move nodeLevelMonitoring under monitoring as well in a future merge request?

Also includes:

• Moving component_node under monitoring https://gitlab.com/gitlab-com/runbooks/-/blob/2ffa9253c5e65731aeaeecf750261c2bd2e759c9/metrics-catalog/services/gitaly.jsonnet#L24-26
• Follow up from !6563 (comment 1663290691), to support specifying monitoringThresholds.component_node for both service and SLI level.
• Move the monitoringThresholds (in service level) monitoringThresholds+ (inside SLIs) to the top level monitoring.global object. Context: !6394 (comment 1589253395)