feat: Allow override of domain metadata attribute

[qhanam]

In #430, we propose allowing all metadata attributes to be overridden. The primary motivation for this change is to work around the fact that CloudWatch RUM does not currently support associating multiple domains with an app monitor. Specifically, by allowing applications to overwrite the domain read from window.location, the application can spoof the domain and pass the domain check in CloudWatch RUM.

However, allowing all metadata attributes to be overridden could make it more difficult to provide backwards compatibility in the future, or cause users pain if they accidentally override a built-in metadata value.

For example, if the user agent is parsed by the service, instead of here, the service may choose overwrite the metadata values provided in the request. A counter argument is that allowing metadata attributes to be overridden makes it easier for applications to set values in cases where the built-in metadata values are incorrect.

Instead, this change allows only the domain attribute to be overridden.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[williazz]

Question: so just so I fully understand, Cx can do something like this to use multiple apps to the same app monitor?

cwr('addSessionAttributes', {
    domain: 'override', // spoof
    trueDomain: window.location.origin //  for actual tracking
}) 

[qhanam]

just so I fully understand, Cx can do something like this to use multiple apps to the same app monitor?

Correct. Alternatively, a RUM user could differentiate user flows across domains by adding the domain to the page Id.