Mongodb input connection metrics

[tuapuikia]

Relevant telegraf.conf:

System info:

Telegraf version: nightly version (mater 54f28ee)
Input plugin : mongodb
Enabled : gather_perdb_stats = true

Steps to reproduce:

1. Configure mongodb shard cluster (3 replica set with 1 primary + 2 secondary)
2. Collect mongodb metric using telegraf --config /etc/telegraf/telegraf.conf --test

Expected behavior:

1. Connection metric available in connections_available, connection_current and connections_total_created.

Actual behavior:

1. No value exported. Only 0 printed.

Additional info:

1. The new metric are only available in nightly build but not working.

[Include gist of relevant config, logs, etc.]