align jmx state metrics with semconv

[SylvainJuge]

Fixes #14753

• update feature description and link to semconv
• change unit to 1 where it was previously "" (empty string)

In the existing semantic conventions, there are at least two examples of "state metrics" that already use 1 as unit, so it is probably simpler to align to semconv than try to change semconv to use an empty metric.

• hw.status (network)
• hw.network.up

Only the state metric unit changes, so while it's technically a breaking change I think the impact should be quite limited:

• 1 as unit is used mostly for fractions of something, usually with values between 0.0 and 1.0.
• Because current empty unit for state metrics is used to indicate "lack of unit", the value is unlikely to be used for meaningful purpose. However any tool or mapping relying on the current unit might need some (but minor) adjustment.

So, in the current state I don't think this requires to have a dedicated configurable toggle to leave this breaking change for 3.0, however I don't mind taking the "extra safe" way and add a config option just to make the transition smoother.

In addition, due to how the feature is currently built, the configuration options are not automatically propagated to usages of the library like in the jmx-scraper so adding a new configuration option would require to also implement it in every usage of the library, then deprecate and revert the change. This inconvenience might be removed once #14674 is completed, but I'm not 100% sure about that, so in the mean time it's probably simpler to deal with what appears to be a minor breaking change.