Adds hists for MarathonStore read-data-size, write-data-size.

State-related performance metrics are now the following:

```
"mesosphere.marathon.state.AppRepository.read-request-time"
"mesosphere.marathon.state.AppRepository.write-request-time"
"mesosphere.marathon.state.DeploymentRepository.read-request-time"
"mesosphere.marathon.state.DeploymentRepository.write-request-time"
"mesosphere.marathon.state.GroupRepository.read-request-time"
"mesosphere.marathon.state.GroupRepository.write-request-time"
"mesosphere.marathon.state.MarathonStore.AppDefinition.read-data-size"
"mesosphere.marathon.state.MarathonStore.AppDefinition.write-data-size"
"mesosphere.marathon.state.MarathonStore.DeploymentPlan.read-data-size"
"mesosphere.marathon.state.MarathonStore.DeploymentPlan.write-data-size"
"mesosphere.marathon.state.MarathonStore.EventSubscribers.read-data-size"
"mesosphere.marathon.state.MarathonStore.EventSubscribers.write-data-size"
"mesosphere.marathon.state.MarathonStore.Group.read-data-size"
"mesosphere.marathon.state.MarathonStore.Group.write-data-size"
"mesosphere.marathon.state.MarathonStore.TaskFailure.read-data-size"
"mesosphere.marathon.state.MarathonStore.TaskFailure.write-data-size"
"mesosphere.marathon.tasks.TaskTracker.read-request-time"
"mesosphere.marathon.tasks.TaskTracker.write-request-time"
"mesosphere.marathon.state.AppRepository.read-request-errors"
"mesosphere.marathon.state.AppRepository.read-requests"
"mesosphere.marathon.state.AppRepository.write-request-errors"
"mesosphere.marathon.state.AppRepository.write-requests"
"mesosphere.marathon.state.DeploymentRepository.read-request-errors"
"mesosphere.marathon.state.DeploymentRepository.read-requests"
"mesosphere.marathon.state.DeploymentRepository.write-request-errors"
"mesosphere.marathon.state.DeploymentRepository.write-requests"
"mesosphere.marathon.state.GroupRepository.read-request-errors"
"mesosphere.marathon.state.GroupRepository.read-requests"
"mesosphere.marathon.state.GroupRepository.write-request-errors"
"mesosphere.marathon.state.GroupRepository.write-requests"
"mesosphere.marathon.tasks.TaskTracker.read-request-errors"
"mesosphere.marathon.tasks.TaskTracker.read-requests"
"mesosphere.marathon.tasks.TaskTracker.write-request-errors"
"mesosphere.marathon.tasks.TaskTracker.write-requests"
```