fix derived rate, fixes #20097

[wburgers]

Description:

The rate (m3/h) was calculated as current_reading - previous_reading. So time was not considered.
This PR fixes the rate by calculating the time between the readings and converting that to m3/h.

Related issue (if applicable): fixes #20097

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#<home-assistant.io PR number goes here>

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Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

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[homeassistant]

Hi @wburgers,

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Once you do that we will be able to review and accept this pull request.

Thanks!

[amelchio]

Sorry for clicking around, in fact I had one more comment.

[amelchio]

Great, thank you 🎉

[leeuwte]

by chance I added this component as a custom_component to already use my changes in advance. But I'm getting an error after this PR;

Traceback (most recent call last):
  File "/usr/src/app/homeassistant/helpers/entity.py", line 221, in async_update_ha_state
    await self.async_device_update()
  File "/usr/src/app/homeassistant/helpers/entity.py", line 347, in async_device_update
    await self.async_update()
  File "/config/custom_components/sensor/dsmr.py", line 354, in async_update
    self._state = diff / timediff * 3600
TypeError: unsupported operand type(s) for /: 'decimal.Decimal' and 'datetime.timedelta'

[amelchio]

@leeuwte Thanks for testing 👍. Please file a GitHub issue to track this regression.