More Metrics Collector tests

[gauravIoTEdge]

This PR adds more tests for verifying the AllowedMetrics filters in the existing Metrics Collector.

Testing Notes

Automated tests

• New tests added & existing tests pass.

Manual testing/verification

1. Deployed existing IoTEdgeMetricsCollector from Azure Marketplace.
2. Built new metrics collector with the fix from this PR: Bug fix: Filtering metrics does not work if endpoint is specified. #5665
3. New deployment ran modules from steps 1&2, for the same set of filters testing the same built-in metrics.
4. Tested & verified that existing regex/label matching for AllowedMetrics works as expected by looking at the logs for both modules (from steps 1 & 2).

Potential testing limitations

• Manual tests did not cover multiple/nested device scenarios.
• However, an effort has been made to emulate such scenarios in the automated tests.

[x] I have read the contribution guidelines.