Overhaul stats: Scaffolding for metrics in the tracker-core package

[josecelano]

Relates to: #1502

It adds a new metric.

The new metric in Prometheus format:

tracker_core_persistent_torrents_downloads_total{} 1

In new PRs, the new metric will be persisted.

[codecov]

Codecov Report

Attention: Patch coverage is 39.71631% with 85 lines in your changes missing coverage. Please review.

Project coverage is 83.52%. Comparing base (f21957d) to head (2175270).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
packages/tracker-core/src/statistics/repository.rs	25.37%	50 Missing ⚠️
packages/tracker-core/src/statistics/metrics.rs	24.24%	25 Missing ⚠️
...s/rest-tracker-api-core/src/statistics/services.rs	0.00%	5 Missing ⚠️
...racker-api-server/src/v1/context/stats/handlers.rs	0.00%	2 Missing ⚠️
...racker-client/src/http/client/requests/announce.rs	0.00%	2 Missing ⚠️
src/bootstrap/jobs/tracker_core.rs	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1536      +/-   ##
===========================================
- Coverage    83.89%   83.52%   -0.37%     
===========================================
  Files          270      273       +3     
  Lines        19181    19317     +136     
  Branches     19181    19317     +136     
===========================================
+ Hits         16091    16134      +43     
- Misses        2817     2909      +92     
- Partials       273      274       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[josecelano]

ACK 2175270