Monitoring graphs - Popover value improvements

Follow up for unfinished monitoring graph improvements https://gitlab.com/gitlab-org/gitlab-ce/issues/38030:

Spec files: https://gitlab-org.gitlab.io/gitlab-design/hosted/taurie/ce%2338030-spec-previews

• Added dots to the graph points at the current cursor location

• The font weight should be 600

• The triangle should be placed on the side of the gray header

• The legend format time and date stamp should be formatted as “DD MMM, YYYY 00:00pm”