Expected results:

• Assigning values using `mongodb.collstats-colls` flag only enables collection stats for the listed collections.

• Assigning values using `mongodb.indexstats-colls` flag only enables index stats for the listed collections.

Actual results:

• Assigning values using `mongodb.collstats-colls` flag enables both collection and index stats.

• Assigning values using `mongodb.indexstats-colls` flag has no effect.

IndexStatsCollections is being assigned the values from the `mongodb.collstats-colls` flag. 

Appears to be a bug here: https://github.com/percona/mongodb_exporter/blob/be2da0287d612e01e85e02b2ee920f0f2906407a/main.go#L98

HOW TO TEST:

1. Create a collection in MongoDB.

2. Add an index to the collection. 

3. Manually start the exporter specifying the collection.index

4.  Browse to the exporter's URL and you must see stats regarding the index usage.

5. CollStats shouldn't be enabled.

Example in mongo shell

Then if you start the exporter like this: 

/mongodb_exporter --mongodb.uri=mongodb://127.0.0.1:17001/admin --mongodb.indexstats-colls=test.c1

and browse to http://localhost:9216/metrics
you will see something like this:  (search for test.c1, the db.collection name)