Metrics query times out with error suggesting "Please consider making the query more selective"

Problem:

When running a metrics query like "_sourceCategory="prod/metrics" over a 5 day time range, we are seeing this error message:

The metrics query timed out. Please consider making the query more selective.

Cause:

When a metric query returns more than 300 time series, Sumo presents a message like the one above. 

Resolution:

Add additional query selectors or parameters to make the query more selective and reduce the number of time series returned by the metrics query. This will make the visualization more effective and understandable. The doc link provides more details - but included below is a snippet.

As you start typing, the available choices are displayed.  

Follow these guidelines to complete your queries:

• Select from the listed options or type directly into the text area.
• To see all of the matching tags, values, or metrics, select View All Tags, View All Values, or View All Metrics. A dialog box opens for you to make a selection that matches, including any text that you've already entered. Scroll or enter text in the Search field to narrow down your search, and click Add to Query when you've made a selection.
  
   
• Specify multiple pairs within a query for an AND, OR, or NOT/! match. If you enter multiple pairs manually, make sure to include a space between tag=value pairs. For example, this query matches HostMetrics AND the instance identifier i-e0b45532:
  _contentType=HostMetrics InstanceId=i-e0b455532