Detect emptied nos contact lists + metrics

[dcadenas]

This is one of the pending tasks listed in https://github.com/planetary-social/nos-followers/issues/1.

Implements a metric and alert based on the iOS issue to tag the nos client during contact lists updates. I assumed that the tag will be nos but we can always change that. I also added a new metric to detect when we ignore a contact list because we have a fresher version.

Pending work:

• [ ] Integrate these metrics into grafana and create alert for Slack notification

[coveralls]

Pull Request Test Coverage Report for Build 10816463729

Details

• 27 of 35 (77.14%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 69.666%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/domain/follows_differ.rs	21	23	91.3%
src/metrics.rs	6	12	50.0%
<!--	Total:	27	35	77.14%	-->

Totals
Change from base Build 10816339215:	0.06%
Covered Lines:	1281
Relevant Lines:	1427

---

💛 - Coveralls