Fix mongodb_connections metric for mongos instances

[vtomasr5]

The getDiagnosticData command returns empty data for mongos, causing connection metrics to be missing. This adds a fallback to serverStatus when getDiagnosticData returns empty data for mongos instances.

This ensures that mongodb_connections (and mongodb_ss_connections) metrics are properly exposed for mongos routers, maintaining compatibility with monitoring dashboards and alerting rules.

May fix #916

[it-percona-cla]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.77%. Comparing base (dc46ed5) to head (5086741).
⚠️ Report is 135 commits behind head on main.

Files with missing lines	Patch %	Lines
exporter/diagnostic_data_collector.go	60.00%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1151      +/-   ##
==========================================
- Coverage   70.88%   68.77%   -2.12%     
==========================================
  Files          28       29       +1     
  Lines        3569     3110     -459     
==========================================
- Hits         2530     2139     -391     
+ Misses        904      799     -105     
- Partials      135      172      +37     

Flag	Coverage Δ
agent	68.77% <60.00%> (-2.12%)	⬇️

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[idoqo]

Thanks a lot for your contribution @vtomasr5 .

If you're still up, can we add some tests for mongos instances? Maybe something similar to TestAllDiagnosticDataCollectorMetrics but connected to the mongos instance instead - only checking for mongodb_ss_connections_* is fine.

Additionally, you can sync this branch with current main and it should fix the duplicate metrics errors in CI.