chore: use gauge for capturing pending events

[lvrach]

Description

Count was the incorrect metric type to capture the number of pending events. Since we were interested in the time value and not the rate of change.

Notion Ticket

https://www.notion.so/rudderstacks/Reduce-alert-noise-Improve-observability-5beff5b1a6e34a92a071b37ab117bf45?pvs=4

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: 52.02% // Head: 51.99% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (1ed2403) compared to base (f20a6f3).
Patch coverage: 6.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2960      +/-   ##
==========================================
- Coverage   52.02%   51.99%   -0.04%     
==========================================
  Files         324      324              
  Lines       51231    51284      +53     
==========================================
+ Hits        26654    26665      +11     
- Misses      22966    23010      +44     
+ Partials     1611     1609       -2     

Impacted Files	Coverage Δ
warehouse/client/controlplane/client.go	34.48% <0.00%> (-26.75%)	⬇️
warehouse/validations/validate.go	51.60% <0.00%> (-2.31%)	⬇️
gateway/webhook/webhook.go	53.70% <66.66%> (+0.11%)	⬆️
warehouse/warehouse.go	11.54% <100.00%> (ø)
router/router.go	76.89% <0.00%> (+0.18%)	⬆️
config/backend-config/namespace_config.go	70.00% <0.00%> (+3.00%)	⬆️
router/manager/manager.go	100.00% <0.00%> (+3.29%)	⬆️

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