Add tag for node type in mongodb input plugin

[rlodge]

Feature Request

Right now, the mongodb input plugin inputs NodeType (PRI, etc) as the member_status field. This isn't as useful as a tag for querying in InfluxDB, so I propose adding a tag with this information as well.

Proposal:

Add a tag, node_type, to the output of the mongodb input plugin.

Current behavior:

A field member_status is added

Desired behavior:

Both a field member_status and a tag node_type would be added.

Use case:

To make it easy to distinguish between different node types when querying, or to group by different node types when querying, e.g. to show only stats from primary nodes.