Fix gas energy graph units if stats added by external source

[pszafer]

Proposed change

By default Energy dashboard get units from entity.
If stats for gas are added by async_add_external_statistics then entity don't exists, so energy dashboard is falling back to m³ units.
This PR fixes this issue and get unit from statistic_meta if entity doesn't exists.

Depends on core PR home-assistant/core#68471

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

[emontnemery]

I think it's correct to only check the statistic meta data, instead of checking the state first and falling back to statistic meta data: while the unit is allowed to change in the state, the unit in the statistics meta data is locked.

Also, getEnergyGasUnitCategory should probably be updated in the same way as getEnergyGasUnit?

[pszafer]

getEnergyGasUnitCategory filters out entities to show in Add source dialog.
For external stats it filters out one level lower between m³, ft³, kWh`, so I think it's ok.

About correct way for filter out entities I can remove this if there is no reason to fetch units from entity itself.

[emontnemery]

Looks good to me.
This should be followed up with another PR replacing existing calls to history/list_statistic_ids with recorder/get_statistics_metadata.