fix: unnecessary router pending event counts

[Sidddddarth]

Description

Fixes unnecessary extra statistics captured at router level, where pending event Gauges were initialized for all destinations for all workspaces served by the server instance.

Notion Ticket

fix router pending event alerts

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Base: 47.62% // Head: 47.65% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (b8cf5c1) compared to base (0459aff).
Patch coverage: 67.59% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2849      +/-   ##
==========================================
+ Coverage   47.62%   47.65%   +0.03%     
==========================================
  Files         311      311              
  Lines       49687    49708      +21     
==========================================
+ Hits        23663    23689      +26     
+ Misses      24580    24578       -2     
+ Partials     1444     1441       -3     

Impacted Files	Coverage Δ
router/batchrouter/batchrouter.go	37.53% <6.25%> (-0.09%)	⬇️
services/multitenant/tenantstats.go	89.24% <12.50%> (-1.63%)	⬇️
processor/processor.go	86.34% <76.78%> (+0.01%)	⬆️
router/router.go	76.69% <100.00%> (+0.23%)	⬆️
jobsdb/migration.go	76.94% <0.00%> (+3.10%)	⬆️

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