Move Metering cluster formatting out of cluster handler and into sensor entity

[puddly]

When working on #557 I ran into a bug: diagnostics could not be regenerated for a specific device. For some reason, the cluster handler-specific initialization code did not run.

The cluster handler computed something that realistically should be handled by the entity so I ported the code over into there. This has the benefit of now allowing the summation formatting to change at runtime.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.91%. Comparing base (76550e0) to head (3745fa4).

Files with missing lines	Patch %	Lines
zha/application/platforms/sensor/__init__.py	93.10%	2 Missing ⚠️
zha/application/platforms/sensor/helpers.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #565      +/-   ##
==========================================
- Coverage   96.95%   96.91%   -0.04%     
==========================================
  Files          63       63              
  Lines       10534    10543       +9     
==========================================
+ Hits        10213    10218       +5     
- Misses        321      325       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.