Support for _FILE secret of mongo-uri

**Is your feature request related to a problem? Please describe.** At present the image only accepts a command or env variable; --mongodb.uri or MONGODB_URI. There is no clean way to pass sensitive information (user and pass) across into the docker container, and the image doesn't have shell baked in so there's no way to manually interpolate a secret. **Describe the solution you'd like** Similar to the [Redis Exporter](https://github.com/oliver006/redis_exporter) and [Postgres Exporter](https://github.com/prometheus-community/postgres_exporter), I would like one or more ENVIROMENT fields using the standard _FILE suffix where docker secrets can be specified e.g. `MONGODB_URI_FILE=/run/secrets/mongo_uri` , which are then automatically picked up by the service internally rather than passing in the data through raw environment variables (unsafe). **Describe alternatives you've considered** The only current solutions are mounting the full URI containing sensitive credentials or using a sidecar container with shell support.