Setup code coverage reporting

Rust Version

stable

Diesel Version

main

Diesel Features

all of them

Database Version

any

Operating System Version

Linux

What happened?

We currently have quite a lot of tests, but we don't 100% know what's covered and uncovered in terms of API surface. We would like to change that

What did you expect to happen?

Our CI setup should be able to report a test coverage for our code base. This likely requires a multi-step procedure as we need to merge coverage for all backends and between normal unit/integration tests and doc-tests to get a full coverage picture.

I'm not sure yet what's the best way to record and report the coverage values itself and their changes.

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.

• This issue can be reproduced on Rust's stable channel. (Please submit the issue in the Rust issue tracker instead if that's not the case)

• This issue can be reproduced without requiring a third party crate