Update attribute processor for stable metrics

[yangjian]

Previously AttribtueProcessor is a class and not open. It can't be customized externally. Now it's changed to procotol and its static functions are moved into SimpleAttributeProcessor (now it's public).

Also duplicated NoopAttributesProcessor is removed and more tests are added.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: yangjian / name: Yang Jian (6e5ad4b)
• ✅ login: nachoBonafonte / name: Ignacio Bonafonte (7a271ed)

[nachoBonafonte]

I would like to understand the use case for this change, whyy the need to inherit AttributeProcessor instead of extending it

[yangjian]

There're common labels for all metrics in our iOS app. Attribute processor can be used to merge them into final labels. But one of these labels can be changed by user or server. Customized attribute processor can be more flexible to support it. Something like this:

class GlobalSharedLabels: AttributeProcessor {
    let attributes: ManagedAtomic<AtomicContainer<[String: AttributeValue]>>

    public updateLabels(...) {
        attributes.store(...)
    }

    public func process(incoming: [String: AttributeValue]) -> [String: AttributeValue] {
        incoming.merging(attributes.load(...), uniquingKeysWith: { $1 })
    }
}

Alternatively, it can return a SimpleAttributeProcessor, but it isn't exported before.

[nachoBonafonte]

LGTM, I had some doubts at the beginning but keeping the base code as part of the protocol with an extension makes total sense. Thanks a lot for your contribution,

[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 8 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@4d1502b). Click here to learn what that means.

Files	Patch %	Lines
...TelemetrySdk/Metrics/Stable/View/ViewBuilder.swift	0.00%	5 Missing ⚠️
...rySdk/Metrics/Stable/View/AttributeProcessor.swift	85.71%	3 Missing ⚠️

Additional details and impacted files

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  Coverage        ?   67.45%           
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  Files           ?      335           
  Lines           ?    14585           
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  Hits            ?     9839           
  Misses          ?     4746           
  Partials        ?        0           

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