Add metric for agencies with coverage

[aaronbrethorst]

Note: Blocked on #3

User story

as a devops engineer,
I want to make sure that all agencies in the bundle are represented in the server,
so that I can be confident about the state of my data

Acceptance criteria

• add a new metrics file/function at internal/metrics/agencies_with_coverage.go / CheckAgenciesWithCoverage(), and wire it up in api/metrics.go
• Verify that the number of agencies represented in the static GTFS file matches the number of agencies in the agencies-with-coverage endpoint.
• Add tests verifying that this works as expected