feat(gax)!: preserve retry loop errors

[coryan]

Fixes #2432

[dbolduc]

Returning LoopState

Pros:

• applications do not need to learn a new type

Cons:

• permanent should never happen, and could be disallowed

I am not opposed to returning this type but wanted to call this out. Thoughts?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.51%. Comparing base (26e9a46) to head (e8546d5).
Report is 34 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2456      +/-   ##
==========================================
+ Coverage   95.39%   95.51%   +0.12%     
==========================================
  Files          77       79       +2     
  Lines        3060     3168     +108     
==========================================
+ Hits         2919     3026     +107     
- Misses        141      142       +1     

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[dbolduc]

I feel like attempt_count does not tell us enough, when throttles don't increase attempts.

But I also remember that the interface is hidden, and not public. This PR is about preserving the last error, which it does. So approved and we can nit about the interface later.

[coryan]

PTAL

[dbolduc]

👍