Retrieve bounce queue with the postfix input plugin

## Feature Request

Retrieve bounce queue with the postfix input plugin and sum the total mail queued.

### Proposal:

- Retrieve bounce queue with the postfix input plugin
- Add a fake queue named "total"

### Current behavior:

Currently not all postfix queues are retrieved :
queue=active
queue=hold
queue=incoming
queue=maildrop
queue=deferred

### Desired behavior:
queue=active
queue=hold
queue=incoming
queue=maildrop
queue=deferred
**queue=bounce
queue=total**

### Use case:

I use a custom collector, but it would be nice to switch to the telegraf native input plugin.

Locate the Postfix input plugin and read how it currently retrieves the active, hold, incoming, maildrop, and deferred queues. Trace the plugin's existing metric output and tests, if present. Done means the plugin exposes a bounce queue and a total queue representing the requested aggregate.

Written by the indexing model from the issue text.