Feature Request: Expose database mode in maxminddb.reader.Metadata

[nickwilliams-eventbrite]

In production environments, it can be hard to determine which mode (MODE_FILE, MODE_MMAP, MODE_MMAP_EXT, etc.) the reader is using with MODE_AUTO without experimenting (setting the mode explicitly), seeing which modes work, and then assuming MODE_AUTO is selected the mode you want (MODE_MMAP_EXT). Experimenting in production can cause problems.

It would be very helpful if the maxminddb.reader.Metadata object exposed the mode (string value) and whether that mode was auto or explicit (boolean). That way, a simple glance at the metadata could confirm that you've installed and configured everything correctly.