Add markers on charts

[edugfilho]

Markers will be useful to have on charts, so users don't get so confused when certain things happen. Non comprehensive list of examples:

• A distribution became labeled and there are no older values for the new labels
• An aggregation type changed (see labeled_counter metrics use sample counts instead of counter values #3524)
• A problematic build has an unusually low amount of clients
• We permanently lost data upstream for certain build(s)

┆Issue is synchronized with this Jira Task