feat: Add metric for Store And Forward queue size

[lenny-goodell]

closes #1138

If your build fails due to your commit message not passing the build checks, please review the guidelines here: https://github.com/edgexfoundry/app-functions-sdk-go/blob/main/.github/CONTRIBUTING.md

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• [x] I am not introducing a new dependency (add notes below if you are)
• [ ] I have added unit tests for the new feature or bug fix (if not, why?)
• [ ] I have fully tested (add details below) this the new feature or bug fix (if not, why?)
• [ ] I have opened a PR for the related docs change (if not, why?)

Testing Instructions

New Dependency Instructions (If applicable)

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (15d59e2) 66.27% compared to head (11d2ee9) 66.19%.

Files	Patch %	Lines
internal/runtime/storeforward.go	50.00%	4 Missing and 2 partials :warning:

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1538 +/- ## ========================================== - Coverage 66.27% 66.19% -0.09% ========================================== Files 36 36 Lines 3238 3248 +10 ========================================== + Hits 2146 2150 +4 - Misses 951 955 +4 - Partials 141 143 +2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.