Add interval duration to history tooltip

[brunoluvizotto]

Proposed change

When I'm checking entities history, sometimes I would like to know the duration of certain interval and I have to hover over the interval to see the tooltip and get the start and end dates to do the math.

This PR adds the duration to the tooltip.

Before

After

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:
• 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