Updated chart series props parsing

Problem

Series options passed as props to charts in Monitor dashboard are objects and not array. Because of this the change that went in !438 (merged) would break the monitor dashboard.

This issue was brought to attention here https://gitlab.slack.com/archives/C0GQHHPGW/p1562083270478200 (internal slack chat)

Lukas 'Eipi' Eipert: In the recent GitLab UI update (5.1.0 -> 5.2.0) some tests are failing. Could someone have a look? https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30259

Solution

This MR updates the logic to parse chart series options to accommodate series passed as an object or array of objects.