Improve test coverage for prometheus application plugin (#166)

* Improve test coverage for prometheus application plugin

Add tests for New, Config, PromCollector methods, metricExpiry,
collectorExpiry, ReceiveMetric, UpdateMetrics, Describe, and Collect.
Omit logging and HTTP server tests.

* Fix golangci-lint empty-block error in prometheus tests

Replace empty for-range loop with explicit assignment to satisfy revive linter.

* Add timestamp verification assertions to prometheus collector tests

Enhance test coverage by adding assertions that verify the presence or
absence of timestamps in collected metrics. The "collect with timestamp"
test now verifies that metrics include the expected timestamp value,
while the "collect with zero timestamp" test confirms that metrics with
zero timestamps are sent without timestamp metadata.

Also fixes default port assertion and cleans up empty-block lint issue.

* Fix golangci-lint empty-block error in channel drain loop

Replace empty for-range loop with explicit variable usage to resolve
the revive empty-block linting error.

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: vkmc