Add metrics tests

[johnfercher]

Description

Related Issue

• Add metrics tests

Checklist

check with "x", if applied to your change

• All methods associated with structs has func (s *struct) method() {} name style.
• Wrote unit tests for new/changed features.
• Updated docs/doc.go
• Updated pkg/pdf/example_test.go
• Updated README.md
• Updated all examples inside internal/examples
• New public methods/structs/interfaces has comments upside them explaining they responsibilities
• Executed make dod with none issues pointed out by golangci-lint and goreportcard-cli

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (v2@6197ee3). Click here to learn what that means.

Additional details and impacted files

@@          Coverage Diff          @@
##             v2     #354   +/-   ##
=====================================
  Coverage      ?   71.24%           
=====================================
  Files         ?       61           
  Lines         ?     1999           
  Branches      ?        0           
=====================================
  Hits          ?     1424           
  Misses        ?      551           
  Partials      ?       24           

Flag	Coverage Δ
unittests	71.24% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

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