chore: add additional metrics (websockets / node stats)

[darrenvechain]

Description

Adding metrics around subscriptions and node stats

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

TODO

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] New and existing E2E tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 62.06897% with 11 lines in your changes missing coverage. Please review.

Project coverage is 62.13%. Comparing base (6c62498) to head (e003a86).

Files	Patch %	Lines
metrics/prometheus.go	20.00%	4 Missing :warning:
cmd/thor/node/node.go	0.00%	3 Missing :warning:
cmd/thor/node/packer_loop.go	0.00%	2 Missing :warning:
metrics/noop.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #778 +/- ## ========================================== - Coverage 62.14% 62.13% -0.01% ========================================== Files 203 203 Lines 18688 18702 +14 ========================================== + Hits 11613 11620 +7 - Misses 5975 5980 +5 - Partials 1100 1102 +2 ```

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[darrenvechain]

@otherview screenshots of the affected metrics: