[ML] Single metric viewer doesn't work if bucket span is a multiple of a week (or higher) unit

[dolaru]

When accessing the Single metric viewer, an elasticsearch request with a date histogram aggregation is fired. The interval of that aggregation is set to the bucket span using the highest matching time unit.

According to the Date Histogram Aggregation documentation:

... note that time intervals larger than than days do not support arbitrary values but can only be one unit large (e.g. 1y is valid, 2y is not).

As a result, if the bucket span is a multiple of a unit higher than days, the Elasticsearch search request fails.

Example

Job with bucket span of 28d, when viewed in the Single Metric Viewer will try to fire an Elasticsearch search request with a date histogram aggregation which has an interval of 4w which is invalid.

Proposed solution

Limit used interval units to not be higher than days.

[elasticmachine]

Pinging @elastic/ml-ui

[jgowdyelastic]

The issue appears to be with the use of moment's .expression which turns 28d into 4w when creating the search.
e.g.
https://github.com/elastic/kibana/blob/master/x-pack/plugins/ml/public/timeseriesexplorer/timeseriesexplorer_controller.js#L272

A possible fix would be to use asSeconds or asMilliseconds or whatever elasticsearch's default unit is.