Add lustre_connected_clients metrics for OST & MGS

[RDruon]

Expose lustre_connected_clients for OST and MGS.

Draft as MGS will include server connection, looking for a better way to handle it.

[codecov]

Codecov Report

Attention: Patch coverage is 99.42363% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.12%. Comparing base (05ccd69) to head (ae2585e).

Files with missing lines	Patch %	Lines
lustre-collector/src/mgs/client_count_parser.rs	99.24%	0 Missing and 1 partial ⚠️
lustre-collector/src/oss/client_count_parser.rs	99.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
+ Coverage   91.67%   92.12%   +0.44%     
==========================================
  Files          42       44       +2     
  Lines        5792     6135     +343     
  Branches     5792     6135     +343     
==========================================
+ Hits         5310     5652     +342     
  Misses        435      435              
- Partials       47       48       +1     

Flag	Coverage Δ
2_14_0_ddn133	34.39% <20.11%> (-1.30%)	⬇️
2_14_0_ddn145	35.34% <20.11%> (-1.38%)	⬇️
all-tests	92.12% <99.42%> (+0.44%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ 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.

[github-actions]

Benchmark for 1c9f14f

Click to view benchmark

Test	Base	PR	%
jobstats otel 100	1739.2±41.59µs	1745.5±45.19µs	+0.36%
jobstats otel 1000	16.0±0.13ms	16.1±0.20ms	+0.62%

[notion-workspace]

Add lustre_connected_clients metrics for OST & MGS

[github-actions]

Benchmark for c391fd9

Click to view benchmark

Test	Base	PR	%
jobstats otel 100	1705.8±22.79µs	1722.3±32.94µs	+0.97%
jobstats otel 1000	15.9±0.09ms	16.0±0.28ms	+0.63%