Upgrade EstimatorResult dataclass

[pedrorrivero]

Summary

Upgrades the EstimatorResult dataclass.

Breaking change*
Closes #8100
Changelog: New Feature

Details and comments

• Rename values field to expectation_values.
• Add variances field, pulling it out of metadata.
• Remove numpy array type in favor of core python tuple.
• Change metadata list type (mutable) in favor of tuple (immutable).

*Assuming that these objects are meant to be consumed by the users, not instantiated, the only breaking change is the metadata type change from list to tuple; since we can alias expectation_values to the old name values with the appropriate type casting.

To do

• Update Estimator class.
• Fix and update tests.
• Add release note.

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[jakelishman]

I'm not involved in this module at all, so my view isn't especially important, I just wanted to point out that making breaking changes to an API is generally not something we allow within Qiskit. We have a complete deprecation policy, but extensible APIs that other people are meant to implement can't really be changed without some form of versioning, because there's not usually a way to make things valid for two different versions of Terra at the same time.

But do note that I'm not involved with primitives at all, so I'm not a reviewer for this change, and welcome to the org!

[t-imamichi]

We put variance in metadata because we have not concluded which information to add in addition to the expectation value, variance, standard deviation or standard error. We currently use metadata because we can modify it without changing the interface. If we put the additional information as the attribute, we have to choose which to include, variance, standard deviation or standard error.

[coveralls]

Pull Request Test Coverage Report for Build 2385783224

• 13 of 13 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 84.38%

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Totals
Change from base Build 2371064921:	0.008%
Covered Lines:	54548
Relevant Lines:	64646

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💛 - Coveralls

[pedrorrivero]

Has any decision been reached already @t-imamichi ?

[t-imamichi]

I'm afraid that we don't have discussion about metadata yet.

[pedrorrivero]

Outdated, closing and starting anew.