Add note about adapter_response in run_results.json, sources.json

[jtcohen6]

Description & motivation

(reflects ongoing work in dbt-labs/dbt-core#2961 and future work proposed by dbt-labs/dbt-core#2964)

adapter_response is going to be the best place for users to grab out database-specific info like:

• How many rows were created / inserted / updated / merged by this model run?
• [BigQuery] How many bytes were processed?

Today, it is populated for all "true" materializations (model runs, seeds, snapshots), but it's left empty by dbt test and dbt source snapshot-freshness. We'd like to change that in a future release; the immediate benefit is for BigQuery users who want to know the bytes processed / $$ of tests and freshness checks.

Also, while today it only reflects the "last" query executed within a materialization, we'd love someday to record a response for each step in multi-step materializations (e.g. incremental models with create temp + delete + insert).

Pre-release docs

Is this change related to an unreleased version of dbt?

• Yes: please
  • update the base branch to next
  • add Changelog components: <Changelog>[New/Changed] in v0.x.0</Changelog>
  • add links to the "New and changed documentation" section of the latest Migration Guide
• No: please ensure the base branch is current
• Unsure: we'll let you know!

[clrcrl]

LGTM!