mod_http_admin_api: metrics: Filter out a value that is commonly nan at startup The upload bytes count is typically nan at startup, which cannot legally be encoded in JSON. I haven't assessed whether any other metrics might emit nan under other circumstances, but this fixes the most visible issue right now.

This module helps make BOSH and WebSocket connection endpoints
discoverable via the HTTP method described in
[XEP-0156](https://xmpp.org/extensions/xep-0156.html#http).