MongoDB: Add cursor Metrics

[grubernaut]

Fixes: #3480

$ GOCACHE=off go test -v -tags=integration ./...
=== RUN   TestAddNonReplStats
--- PASS: TestAddNonReplStats (0.00s)
=== RUN   TestAddReplStats
--- PASS: TestAddReplStats (0.00s)
=== RUN   TestAddWiredTigerStats
--- PASS: TestAddWiredTigerStats (0.00s)
=== RUN   TestAddShardStats
--- PASS: TestAddShardStats (0.00s)
=== RUN   TestAddShardHostStats
--- PASS: TestAddShardHostStats (0.00s)
=== RUN   TestStateTag
--- PASS: TestStateTag (0.00s)
=== RUN   TestGetDefaultTags
--- PASS: TestGetDefaultTags (0.00s)
=== RUN   TestAddDefaultStats
--- PASS: TestAddDefaultStats (0.01s)
PASS
ok      github.com/influxdata/telegraf/plugins/inputs/mongodb   0.012s

$ ./telegraf --config ~/tele-conf --test
> mongodb,host=foo,hostname=127.0.0.1:27017 active_reads=1i,active_writes=0i,commands_per_sec=4i,cursor_no_timeout=0i,cursor_pinned=0i,cursor_timed_out=0i,cursor_total=0i,deletes_per_sec=0i,flushes_per_sec=0i,getmores_per_sec=0i,inserts_per_sec=0i,jumbo_chunks=0i,net_in_bytes=639i,net_out_bytes=27185i,open_connections=2i,percent_cache_dirty=0,percent_cache_used=0,queries_per_sec=2i,queued_reads=0i,queued_writes=0i,resident_megabytes=66i,total_available=0i,total_created=0i,total_in_use=0i,total_refreshing=0i,ttl_deletes_per_sec=0i,ttl_passes_per_sec=0i,updates_per_sec=0i,vsize_megabytes=1013i,wtcache_app_threads_page_read_count=0i,wtcache_app_threads_page_read_time=0i,wtcache_app_threads_page_write_count=22i,wtcache_bytes_read_into=0i,wtcache_bytes_written_from=39880i,wtcache_current_bytes=39003i,wtcache_max_bytes_configured=7778336768i,wtcache_pages_evicted_by_app_thread=0i,wtcache_pages_queued_for_eviction=0i,wtcache_server_evicting_pages=0i,wtcache_tracked_dirty_bytes=0i,wtcache_worker_thread_evictingpages=0i 1525703381000000000

Required for all PRs:

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