mongodb readme and examples

mongodb readme and examples

closes #1039

plugins/inputs/mongodb/README.md

@@ -0,0 +1,53 @@
+# Telegraf plugin: MongoDB
+
+#### Configuration
+
+```toml
+[[inputs.mongodb]]
+  ## An array of URI to gather stats about. Specify an ip or hostname
+  ## with optional port add password. ie,
+  ##   mongodb://user:auth_key@10.10.3.30:27017,
+  ##   mongodb://10.10.3.33:18832,
+  ##   10.0.0.1:10000, etc.
+  servers = ["127.0.0.1:27017"]
+```
+
+For authenticated mongodb istances use connection mongdb connection URI
+
+```toml
+[[inputs.mongodb]]
+  servers = ["mongodb://username:password@10.XX.XX.XX:27101/mydatabase?authSource=admin"]
+```
+This connection uri may be different based on your environement and mongodb
+setup. If the user doesn't have the required privilege to execute serverStatus 
+command the you will get this error on telegraf
+
+```
+Error in input [mongodb]: not authorized on admin to execute command { serverStatus: 1, recordStats: 0 }
+```
+
+#### Description
+
+The telegraf plugin collects mongodb stats exposed by serverStatus and few more
+and create a single measurement containing values e.g.
+ * active_reads
+ * active_writes
+ * commands_per_sec
+ * deletes_per_sec
+ * flushes_per_sec
+ * getmores_per_sec
+ * inserts_per_sec
+ * net_in_bytes
+ * net_out_bytes
+ * open_connections
+ * percent_cache_dirty
+ * percent_cache_used
+ * queries_per_sec
+ * queued_reads
+ * queued_writes
+ * resident_megabytes
+ * updates_per_sec
+ * vsize_megabytes
+
+
+