Create guideline for modeling state and phase as metrics

[ChrsMark]

That was discussed today in SemConv SIG meeting (Nov 4, 2024). It seems that this modeling could fit well #1032 and in #1212. @braydonk will prepare a proposal to put this as generic guidance in Semantic Conventions (thank you Braydon :)). Based on the outcome of this we can unblock the related PRs.

Originally posted by @ChrsMark in #1032 (comment)

ref: #1032 (comment)

[ChrsMark]

I have filed #1668 which follows the discussed guideline. We can use this this to verify the modeling's proposal.