Remove the isFinal flag from the Metric interface

[philipwalton]

This PR fixes #82 by removing the isFinal flag from the Metric interface.

The primary implication of this change is now when calling any of the metric functions with the reportAllChanges argument set to true, the callback will only be invoked if the metric value changes—i.e. the callback won't be invoked when the page is backgrounded or unloading, nor (in the case of getLCP()) will the callback be invoked after user input.

Prior to this change a callback could be invoked without its value changing, but the isFinal flag would flip from false to true.

cc: @addyosmani as this will affect the Web Vitals Extension.

[rviscomi]

Is this a breaking change, and if so are you planning to release it with a new major version number? (assume so, given the v1 branch, but just confirming)

[philipwalton]

Yeah, this is a breaking change. Sorry if it's not clear from this PR, but the linked issue is part of the v1 milestone.