fix(inputs.stackdriver): Handle when no buckets available

[sspaink]

• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

resolve: #11545

The methods to get buckets could return nil, if all buckets returned nil this wasn't being checked which caused the panic reported in the issue. The code was also getting all buckets even though it if the first bucket type (linear buckets) wasn't nil, although it wouldn't use the other buckets so these were wasted requests. Updated the logic to only continue trying the next bucket type if the previous wasn't nil, following the same precedence logic as before. If all buckets are nil then an error is returned. As you can see in the test some metrics are still gathered, this is existing behavior so I didn't change anything there.

[srebhan]

Thanks for the fix @sspaink. Just one small suggestion... Other than that the code looks good.

[telegraf-tiger]

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