We use `Ops Manager 1.6.2.251`

    # mongo
    MongoDB shell version: 2.6.10
    connecting to: test
    > show dbs;
    admin                   (empty)
    alerts                  0.203GB
    apiv3                   0.578GB
    backupagent             0.078GB
    backupalerts            0.078GB
    backupbilling           0.078GB
    backupconfig            0.078GB
    backupjobs              0.078GB
    backuplogs              2.077GB
    backupoplogs            0.203GB
    backupsnapshot          0.078GB
    cloudconf               0.203GB
    local                   0.078GB
    mmsdb                   0.078GB
    mmsdbagenterrors        0.328GB
    mmsdbagentlog           1.078GB
    mmsdbanalytics          0.078GB
    mmsdbautomation         0.578GB
    mmsdbautomationlog      1.578GB
    mmsdbbilling            0.078GB
    mmsdbconfig             1.078GB
    mmsdbdays               0.078GB
    mmsdbhours              0.203GB
    mmsdbjobs               0.078GB
    mmsdbminutes-even       0.203GB
    mmsdbminutes-odd        0.203GB
    mmsdbpings              0.578GB
    mmsdbprofilerstats      0.078GB
    mmsdbprovisioning       0.078GB
    mmsdbprovisionlog       0.203GB
    mmsdbqueues             0.078GB
    mmsdbratelimit          0.078GB
    mmsdbrrdcache           0.078GB
    mmsdbrrdsampling        0.078GB
    mmsdbserverlog          0.203GB
    mongo-distributed-lock  0.078GB

Our MMS DB use 15 GB of 150 GB available. We can't extend the volume.
We don't use MMS for backup. We use mongodump on other node.

> Ops Manager Application Database This database contains application
> metadata used by the Ops Manager Application. The database stores: •
> Monitoring data collected from Monitoring Agents. • Metadata for Ops
> Manager users, groups, hosts, monitoring data, and backup state. For
> topology and specifications, see Ops Manager Application Database
> Hardware.

At the moment we only enabled Monitoring Agent on Prod. But we wish to enable it on all environments, but cleanup metrics data older than 30 days.

In Oracle you can do something like this. This statement can be scheduled jobs with Oracle Scheduler.

     DELETE
      FROM YOUR_TABLE
     WHERE creation_date <= TRUNC(SYSDATE) - 30

 
The same we wish to to with Automation agent DEBUG logs. Howto do this with MongoDB MMS meta databases?

From [FAQ][1]

> What is the data retention policy for Ops Manager? Ops Manager retains
> two distinct types of data: metrics, which describe usage; and
> snapshots, which back up your data.
> 
> Data-retention policies, as defined in the Terms of Service, are
> always subject to change.
> 
> As of this writing, Ops Manager preserves:
> 
> Minute-level metrics for 48 hours. Hourly metrics for 94 days.
> Snapshots according to their retention policy.


  [1]: https://docs.opsmanager.mongodb.com/v1.6/faq/#data-retention