Fix input plugin MongoDB server connection info

[otherpirate]

The MongoDB plugin currently showing the connection information, example:

./telegraf --config telegraf.conf --test
2018-11-27T20:32:59Z I! Starting Telegraf
> mongodb,host=muraritele-01-154333202752 commands_per_sec=8i,connections_available=0i,connections_current=0i,deletes_per_sec=0i,flushes_per_sec=0i,getmores_per_sec=0i,inserts_per_sec=0i,queries_per_sec=2i,resident_megabytes=49i,updates_per_sec=0i,vsize_megabytes=307i 1543350780000000000
> mongodb_db_stats,db_name=admin,host=muraritele-01-154333202752 avg_obj_size=310.7,collections=3i,data_size=3107i,index_size=81920i,indexes=4i,num_extents=0i,objects=10i,ok=1i,storage_size=65536i,type="db_stat" 1543350780000000000
> mongodb_db_stats,db_name=local,host=muraritele-01-154333202752 avg_obj_size=1826.3333333333333,collections=1i,data_size=5479i,index_size=36864i,indexes=1i,num_extents=0i,objects=3i,ok=1i,storage_size=36864i,type="db_stat" 1543350780000000000
> mongodb_db_stats,db_name=murari_telegraf,host=muraritele-01-154333202752 avg_obj_size=0,collections=1i,data_size=0i,index_size=4096i,indexes=1i,num_extents=0i,objects=0i,ok=1i,storage_size=4096i,type="db_stat" 1543350780000000000

This PR fix #4664

./telegraf --config telegraf.conf --test
2018-11-27T20:39:44Z I! Starting Telegraf
> mongodb,host=muraritele-01-154333202752 commands_per_sec=8i,connections_available=26197i,connections_current=7i,deletes_per_sec=0i,flushes_per_sec=0i,getmores_per_sec=0i,inserts_per_sec=0i,queries_per_sec=2i,resident_megabytes=49i,updates_per_sec=0i,vsize_megabytes=311i 1543351185000000000
> mongodb_db_stats,db_name=admin,host=muraritele-01-154333202752 avg_obj_size=310.7,collections=3i,data_size=3107i,index_size=81920i,indexes=4i,num_extents=0i,objects=10i,ok=1i,storage_size=65536i,type="db_stat" 1543351185000000000
> mongodb_db_stats,db_name=local,host=muraritele-01-154333202752 avg_obj_size=1826.3333333333333,collections=1i,data_size=5479i,index_size=36864i,indexes=1i,num_extents=0i,objects=3i,ok=1i,storage_size=36864i,type="db_stat" 1543351185000000000
> mongodb_db_stats,db_name=murari_telegraf,host=muraritele-01-154333202752 avg_obj_size=0,collections=1i,data_size=0i,index_size=4096i,indexes=1i,num_extents=0i,objects=0i,ok=1i,storage_size=4096i,type="db_stat" 1543351185000000000

As u can see, the connections_current, connections_available and connections_total_created are right after this fix.

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[glinton]

thanks