refactor(metrics): untangle metrics from exporter

[Molter73]

Description

The Exporter type used to own the userspace Metrics object and all other objects would grab the metrics they needed from there. However, because Exporter requires the Bpf object to take the kernel metrics map from it and the Bpf object requires the Exporter to get its userspace metrics from, it created a pretty naste chicken and egg problem. With the addition of the replay feature, this became a bit of a mess.

To clean up the code a bit, Metrics is no longer owned by Exporter, instead an independent instance of Metrics is created and the bits needed by other components are passed to them, then an Exporter is created which registers all metrics from this same object. With this, the new setup_input method can return a tuple with Option<KernelMetrics> and the receiver end of the generated events, which is cleaner than returning the entire exporter.

Checklist

• Patch has a change log entry OR does not need one.
• Investigated and inspected CI test results
• Updated documentation accordingly

Automated testing

• Added unit tests
• Added integration tests
• Added regression tests

If any of these don't apply, please comment below.

Testing Performed

• Check the result of tests for metrics still being recorded as expected.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.17%. Comparing base (a6af5d5) to head (9a68c2c).

Files with missing lines	Patch %	Lines
fact/src/lib.rs	0.00%	27 Missing ⚠️
fact/src/bpf/mod.rs	0.00%	10 Missing ⚠️
fact/src/metrics/mod.rs	0.00%	9 Missing ⚠️
fact/src/metrics/exporter.rs	0.00%	6 Missing ⚠️
fact/src/metrics/kernel_metrics.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                     @@
##           mauro/feat/replay-input    #1118      +/-   ##
===========================================================
- Coverage                    32.29%   32.17%   -0.13%     
===========================================================
  Files                           22       22              
  Lines                         3090     3102      +12     
  Branches                      3090     3102      +12     
===========================================================
  Hits                           998      998              
- Misses                        2089     2101      +12     
  Partials                         3        3              

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