Support reading credentials from file #1288

I'm wondering if, similar to other exporters, the credentials can be passed onto the exporter using files.

For example, in the PostgreSQL exporter, the credentials can be automatically read from a file:
https://github.com/prometheus-community/postgres_exporter#environment-variables

Ideally, this would allow setting the exporter in a monitoring stack like this, using Docker secrets without having to expose the credentials:

    command:
      - --mongodb.uri=mongodb://${MONGO_IP}:${MONGO_PORT}/?authSource=admin
      - --collect-all
    environment:
      MONGO_IP: ...
      MONGO_PORT: ...
      MONGODB_USER_FILE: /run/secrets/mongo_usr
      MONGODB_PASSWORD_FILE: /run/secrets/mongo_pwd
    secrets:
      - mongo_usr
      - mongo_pwd