Only give one validation warning for deprecated cumulative fields

[courtneyholcomb]

Description

These cumulative_type_params validations turned out more annoying than helpful, so we've decided to only give one warning for the entire manifest instead of one per metric. This required a little bit of restructuring.

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have signed the CLA
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [ ] I have run changie new to create a changelog entry