title: Index on ts field in oplog.rs is not updated
body: <p>I have an application that reads the oplog.rs collection in a mongodb 2.2 replica set by querying on the ts field.</p>
  
  <p>I added an index on the ts field, but it doesn't get updated when new entries are inserted into the oplog.</p>
  
  <p>What am I missing? I can't find anything in the mongodb docs about indexes on capped collections are not supported (rather the opposite), and I can't find any info about the oplog being special.</p>
  
  <p>Thanks</p>