Add stats for email tags

### Discussed in https://github.com/devmount/third-stats/discussions/323

<div type='discussions-op-text'>

<sup>Originally posted by **CyrusV5** July  5, 2021</sup>
Hello,
First of all, thank you for creating this tool!
Adding label data to the stats would be useful for people that frequently use labels to sort their emails.</div>