datadog metrics: how to sum after avg?

Question

We have metric called 'crawl':

The tags are:

• 1, stages, values are stage1, stage2, and stage3,
• 2, sources, values are A, B, C
• 3, other tags, like cluster_name, etc

i need to total time spent on stage1, stage2, stage3 for each sources.

i hope to sum the following metrics by stages (stage1+stage2+stage3):

avg:crawl.histogram.95percentile{cluster_name:X} by {stages,sources}

How can i do it? Thanks