Support reading credentials from file

[guillemc23]

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