app/sse: add block processing time metric

[DiogoSantoss]

Add new sseBlockProcessingTimeHistogram metric to track difference between block gossip and head events.

category: feature
ticket: none

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.98%. Comparing base (6426bd7) to head (50670c4).

Files with missing lines	Patch %	Lines
app/sse/listener.go	90.90%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4212      +/-   ##
==========================================
+ Coverage   56.96%   56.98%   +0.01%     
==========================================
  Files         237      237              
  Lines       30622    30651      +29     
==========================================
+ Hits        17445    17465      +20     
- Misses      10945    10952       +7     
- Partials     2232     2234       +2     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud