feat: add tracked object counts metric

[glevco]

Motivation

The Prometheus garbage collector objects metric does not currently reflect the tracked objects count, but a different count according to the docs:

Return the current collection counts as a tuple of (count0, count1, count2).

This PR adds the tracked_objects metric that gets the correct value, and keeps the objects metric with an updated description. Also, for reference.

Acceptance Criteria

• Change HathorSettings.PROMETHEUS_WRITE_INTERVAL from 5 seconds to 15 seconds, as our Prometheus server only scraps the full node every 30s, so we're making too many unnecessary updates.
• Add new Prometheus tracked_objects garbage collector metric.
• Fix Prometheus objects garbage collector metric description and rename it to collection_counts.
• Update GC metrics separately from the other metrics, with a default interval of 60 seconds.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[github-actions]

Bencher Report

Branch	feat/add-gc-metrics
Testbed	ubuntu-22.04

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Benchmark	Latency	Benchmark Result minutes (m) (Result Δ%)	Lower Boundary minutes (m) (Limit %)	Upper Boundary minutes (m) (Limit %)
sync-v2 (up to 20000 blocks)	📈 view plot 🚷 view threshold	1.72 m (-0.74%) Baseline: 1.73 m	1.56 m (90.67%)	2.08 m (82.71%)

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.11%. Comparing base (a54c01c) to head (96c52a6).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
hathor/prometheus.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1247      +/-   ##
==========================================
+ Coverage   86.06%   86.11%   +0.04%     
==========================================
  Files         437      437              
  Lines       33967    33972       +5     
  Branches     5316     5316              
==========================================
+ Hits        29235    29256      +21     
+ Misses       3684     3675       -9     
+ Partials     1048     1041       -7     

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