Add support for accessing Prometheus metrics to celestia-app

Prometheus metrics can be enabled via config files but the port is not yet exposed. We should expose that port and also provide a mechanism to access metrics which can be used from the tests. cc @mojtaba-esk

Start by locating how tastora configures and launches celestia-app, then inspect the existing test setup for ways to access service endpoints. Done means the configured Prometheus port is exposed and tests have a mechanism to retrieve metrics from it.

Written by the indexing model from the issue text.