Export invoice metafields

[kevinpapst]

Description

See #3365

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I verified that my code applies to the guidelines (composer code-check)
• I updated the documentation (see here)
• I agree that this code is used in Kimai and will be published under the MIT license

[codecov]

Codecov Report

Merging #3366 (d778827) into master (452cc46) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #3366      +/-   ##
============================================
+ Coverage     91.84%   91.90%   +0.05%     
  Complexity     7807     7807              
============================================
  Files           742      742              
  Lines         23588    23589       +1     
============================================
+ Hits          21665    21679      +14     
+ Misses         1923     1910      -13     

Impacted Files	Coverage Δ
src/Controller/InvoiceController.php	66.00% <100.00%> (+3.42%)	⬆️
src/Timesheet/DateTimeFactory.php	96.25% <0.00%> (-1.25%)	⬇️
src/Repository/InvoiceRepository.php	54.81% <0.00%> (+2.22%)	⬆️
src/Repository/Loader/InvoiceIdLoader.php	100.00% <0.00%> (+4.00%)	⬆️