[ML] Single Metric Viewer - clarify the time interval a sum or count is over

[pheyos]

Changing the chart zoom in the Single Metric Viewer changes the aggregation interval as well, such that sum and count result in other aggregated values, e.g.:

• aggregation interval of 15m with peak values a bit above 7.000:
  
• same graph with expanded time window such that the aggregation interval is 30m now shows peak values a bit above 11.000 (sum over a longer time gives higher values):
  

The tooltips in the chart show the value of the selected aggregation inteval. To provide some explanation to the user about the changing values in the graph it would be good to add the time interval to the tooltip, like sum over 30m.

[elasticmachine]

Pinging @elastic/ml-ui